Chapter VI: Part II: Marine Invertebrates (1)

I

PORIFERA

(SPONGES)

TABLE SHOWING THE CLASSIFICATION OF THE SPONGES DESCRIBED IN THIS CHAPTER

Phylum and Class =PORIFERA=

Subclass =Calcarea=

Order =HETEROC[OE]LA=

Family ~SYCONIDAE~

Genera Species

^Grantia^ ^G. ciliata^
^Leucosolenia^ ^L. botryoides^

Subclass =Non-Calcarea=

Order =CHONDROSPONGIAE=

Family ~SUBERITIDAE~

^Suberites^ ^S. compacta^
^Polymastia^ ^P. robusta^
^Cliona^ ^C. sulphurea^
Order =CORNACUSPONGIAE=

Family ~DESMACIDONIDAE~

^Esperiopsis^ ^E. quatsinoensis^
^Microciona^ ^M. prolifera^

Family ~HETERORRHAPHIDAE~

^Tedania^

Family ~HOMORRHAPHIDAE~

Genera Species

^Halichondria^ ^H. panicea^

Family ~SPONGIDAE~ Subfamily ~EUSPONGINAE~

^Chalinopsilla^ ^C. oculata^
^C. arbuscula^
^Euspongia^ ^E. officinalis^
Var. ^adriatica^
Var. ^mollissima^
Var. ^rotunda^
Var. ^dura^
^Hippospongia^ ^H. equina^
Var. ^cerebriformis^
Var. ^meandriformis^
Var. ^elastica^
^H. canaliculata^
Var. ^gossypina^
Var. ^flabellum^

Family ~SPONGIDAE~ Subfamily ~STELOSPONGINAE~

^Hircinia^ ^H. campana^

[pg101]

SPONGES

There are many animals which consist of but one cell. These are called /Protozoa/, and comprise the /Infusoria/ and other microscopic organisms. The animals next higher in the scale are /Metazoa/, or multicellular animals, and the first group of this subdivision is =Porifera=, the sponges, the lowest of the many-celled animals.

For a long time sponges occupied a disputed ground between the animal and vegetable kingdoms. Aristotle was the first to point out that a sponge is not a plant. The assertion was doubted and combated, but at last the animal nature of the sponge was established. Sponges were then believed to be colonies of one-celled animals, but finally it was decided that they were individuals with cells of different kinds that performed functions analogous to those of higher organisms.

The sponge, as commonly seen, is only the skeleton or framework, so to speak, of the living animal. In its natural state it is a very different-looking object. Its entire surface is covered with a thin slimy skin, usually of a dark color, which is perforated with holes corresponding to the apertures of the canals. The organic portion of the sponge is a soft, jelly-like substance composed of three layers--the external (/ectoderm/), the internal (/endoderm/), and the middle (/mesoderm/). The external layer is composed of flat cells. The endoderm has cylindrical cells, each one of which has a flagellate hair. The main mass of the body, the mesoderm or middle layer, is made up of cells having various functions, some being concerned in the formation of framework, some in digestion, and some in reproduction.

The framework is secreted in the mesoderm, and in different [pg102] genera consists respectively of a horny or silicious or calcareous substance, or of the first two of these substances combined. The sponge of commerce has the first kind and is composed entirely of exceedingly fine flexible fibers of a horny substance called /spongin/. In other species the spongin is intermixed with spicules of silica, or of carbonate of lime, in various shapes. In the sponges, so much valued as curiosities, called "Venus's flower-basket" and "glass-rope sponge," the framework is composed of silicious spicules alone.

The spicules have a great variety of shapes, being rod-like, knobbed, three-pointed, six-pointed, anchor-like, etc., and are a feature in the classification of sponges.

The sponge is traversed throughout by a canal system, consisting of a series of tubes through which water circulates, carrying air and food to the animal. The exterior of the sponge has numerous small pores and a comparatively few large openings. The fine pores are inhalent, taking in and straining the water of its coarser floating material, and then passing it through perforations in their sides into sacs lined with peculiar cylindrical cells having flagellate hairs, each hair having a collar at its base. These cells, called /choanocytes/, resemble independent animals of the /Protozoa/, known as /flagellate Infusoria/ or /Choanoflagellata/. [pg103] They take in and digest food and eject excrement from the area inclosed by the collar. The cilia (hairs) by their constant movement create currents which keep the water in motion. Water, then, is taken through the pores into the first or incurrent canals; thence it is passed into the ciliated chambers, and thence into the excurrent canals, and out through large passages terminating in large openings called /oscula/, or craters. The canal systems vary. In some species they become quite complex.

Sponges vary greatly in shape, size, color, surface, rigidity, canal systems, and skeleton. They are cake-shaped, tubular, digitate, palmate, cup-shaped, vase-shaped, cone-shaped, spherical, hemispherical, pedunculate, etc., their shapes depending upon whether their growth is uniform or is excessive in a horizontal or in a vertical direction. When they grow evenly in both directions massive uniform shapes arise. If lateral growth predominates, broad, low, and incrusting shapes result. When there is an excess of vertical growth the forms are digitate; [pg104] but if vertical growth is not greatly in excess and is restricted to the marginal part, cup-shaped forms result, and tubular forms when lateral growth is particularly restricted. In some varieties tubular masses coalesce; sometimes flat forms unite in intricate compressed folds. Differences in rapidity of vertical growth make undulations more or less marked, producing lobes and protuberances. Sometimes narrow vertical growth is retarded and horizontal growth predominates, forming various shapes on apparent stems.

Many of the horny sponges are colored, in shades of yellow, brown, red, and violet; some are black.

Sponges are divided into two classes, the =Calcarea= and =Non-Calcarea=. The former have calcareous skeletons, which make them hard and rigid; the latter have skeletons of spongin-fiber or of silicious spicules, or of the two combined.

All sponges, with the exception of one family (the /Spongillidae/, which live in fresh water), are inhabitants of the sea, and live at various depths. The sponges of commerce belong to the /Non-Calcarea/ and are all confined to the genus /Spongia/. In this genus the skeleton is more or less flexible, being composed of spongin. They are shallow-water species, are confined to seas where the waters are of comparatively uniform high temperature, and flourish best when protected by reefs and islands.

There are fisheries for sponges in the waters of the West Indies, the Bahamas, on the southern and western coasts of Florida, and in the Mediterranean and Red seas. Those of the Mediterranean surpass in quality the sponges of our coasts.

Five species of commercial sponges are taken from Florida waters. They are graded by the trade in the order of their importance, as the "sheepswool," "yellow," "grass," "velvet," and "glove." The fishing for sponges is done from small boats, two persons manning each boat. One man sculls, while the other, using a water-glass, scans the bottom. The water-glass is a box with a pane of glass on the bottom. If the glass is held below the surface and the face is placed in the box, the observer is enabled to see with some distinctness the bottom at a considerable depth [pg105] in these clear waters. The sponges are dragged up by hooks--a primitive method which restricts the fishing to shallow water, the fishing here being in water not deeper than thirty feet, but usually three to twenty feet. The sponges are "killed" by being exposed on the beach for several days; they are then placed in "crawls," or pens, where they are washed by the action of the waves for about a week; then, if clean, they are dried, assorted, strung on cords, pressed and baled for shipment. The sponge-fisheries are of considerable value, and much attention is being given to the subject of artificial propagation. It is thought that such beds could be subjected to the regulations which govern oyster-beds. Already the planting of sponges has been shown to be practicable. The living sponge is cut to pieces, and the cuttings are placed in favorable localities. Pieces planted in Florida waters attain a marketable size in one year.

Sponges reproduce by eggs formed in the mesoderm. The eggs escape as ciliated spheres and swim about until they find a place on which to attach themselves. As soon as they become fixed they grow with much rapidity into mature individuals.

Some species seem to prefer association with other animals and live as commensals with crabs. The crab /Dromia/ is always concealed under a sponge, which grows upon its back. Spider-crabs are often overgrown with sponges as well as seaweeds. In this case, however, the crab finds and plants the sponge himself. /Aplysella violacea/ overgrows worm-tubes. Many sponges afford shelter to numerous small animals which bore into their bodies for protection, no animal seeming to feed upon the sponge.

Sponges may be found in tide-pools, on the under side of stones, on seaweeds, and so on. A small bright-red incrusting sponge with irregular lobe-like branches is common on the New England coast; a thin yellow incrusting sponge also is found on the under side of stones. /Grantia ciliata/, a small urn-shaped species, having a large aperture at the summit, is found in tide-pools.

Perhaps the most singular in habit of any sponge is /Cliona sulphurea/, the boring-sponge, a common species found from Cape Cod to South Carolina and abundant in Long Island Sound. It [pg106] is bright sulphur-yellow in color, grows in irregular masses of considerable size and fine texture, and has low wart-like prominences. It lives on shells spreading over both surfaces, at first forming little burrows, but eventually penetrating the shell in every direction, honeycombing and at last completely destroying it by absorption. Sometimes it settles upon living shells and greatly irritates the animal, which constantly secretes new lime to cover the perforations in its shell.

These sponges are an important factor in the economy of the sea, as they disintegrate dead shells, which would otherwise accumulate in vast quantities.

SUBCLASS =CALCAREA=

GENUS ^Grantia^

^G. ciliata.^ Small, urn-shaped or oval, with large aperture at the
summit, surrounded by a circle of projecting spicules. It is found
in tide-pools and on piles of wharves from Rhode Island northward.

GENUS ^Leucosolenia^

^L. botryoides.^ Tubular, branched. Occurs in the same places as
/Grantia ciliata/.

SUBCLASS =NON-CALCAREA=

GENUS ^Suberites^

^S. compacta.^ Elongated, compressed masses, sometimes in several
lobes; attached by one edge; texture fine, firm, compact; surface
smooth; color bright yellow. It grows on sandy bottoms, and is
common in shallow water south of Cape Cod. (Plate XXXVIII.)

GENUS ^Polymastia^

^P. robusta.^ When young it forms yellowish-white incrustations
over shells and stones; later it grows into long, slender, round,
tapering, finger-like projections. Found on the northern New
England coasts in deep water.

GENUS ^Cliona^

^C. sulphurea^, the boring-sponge. Irregular massive form of firm
texture; surface covered with scattered low wart-like prominences
about one eighth of an inch in diameter; bright sulphur-yellow. It
destroys, by absorption, vast quantities of dead shells.

GENUS ^Microciona^

^M. prolifera.^ When young this species forms bright-red
incrustations over shells and stones; later it rises into irregular
lobes and tubular prominences. When fully developed it is profusely
branched in a forking manner. The branches are more or less
flattened, and often are palmate at the ends. It grows in clusters
six inches in diameter, of a dark orange-red color. When dry it is
grayish-brown, brittle, and bristly. It is found from Cape Cod to
South Carolina, and is abundant in Long Island Sound. (Plate XXXIX.)

GENUS ^Tedania^

Irregular, uneven, pale-yellow masses spreading over seaweeds; oscula scattered irregularly over the surface; texture close.

GENUS ^Halichondria^

^H. panicea^, crumb-of-bread sponge. It resembles the crumb of
bread, and is found cast up on all beaches. (Plate XXXVIII.)

GENUS ^Chalinopsilla^

^C. oculata^, the finger-sponge. Stem stout, more or less
flattened, dividing at the upper end into branches which vary in
form and thickness, being finger-like or more or less compressed
lobes; oscula scattered over the smooth, undulating surface;
texture rather hard, but delicate; color, when living, dull
orange-red; when the animal matter is removed, white. The species
is found in shallow and deep water from New York to Labrador.
Common in Massachusetts Bay. (Plate XXXIX.)

^C. arbuscula.^ Profusely branched in a forking manner from close
to the base; branches slender; clusters six to eight inches high
and about the same in breadth; color buff or gray when living,
yellowish-white when free from animal matter; texture finer and
more delicate than that of /C. oculata/. It is found in shallow
water from Cape Cod to North Carolina, and is abundant in Long
Island Sound. (Plate XXXIX.)

GENUS ^Euspongia^

^E. officinalis.^ This is one of the commercial sponges and is
known as the "glove-sponge." It is the one of least marketable
value, having inferior elasticity and becoming brittle with
age; yet a Mediterranean sponge of the same species, variety
/adriatica/, is of the finest quality and greatest value. This
singular fact demonstrates that the quality of sponges depends
largely upon physical conditions. /E. officinalis/ has an average
height of five to six inches. It grows on rocky bottoms in shallow
water on the east coast of Florida. This species has a number of
varieties of various forms; some are dome-shaped, others tubular,
rotund, flabellate, etc. The surface is covered with fine tufts
and is generally free from ridges. On the sides are numerous small
apertures, [pg108] and one or more large oscula occur on top. The
color of the living sponge is black. (Plates XXXIX, XL.)

^E. officinalis^, variety ^adriatica^. More or less globose;
sometimes attached by a broad base, sometimes by a short stem;
latter form more or less club-shaped; oscula scattered over upper
surface. Found in the West Indies and the Mediterranean. (Plate XL.)

^E. officinalis^, variety ^mollissima^, the Levant toilet-sponge.
Generally cup-shaped; oscula on inner side of cup or on upper flat
surface; very soft and elastic.

^E. officinalis^, variety ^rotunda^. Usually massive; attached by a
broad base; sides vertical; oscula large and conspicuous on top, or
small in longitudinal rows on the sides. In the young this variety
may have a conical form with only one orifice, but later it has
several oscula. Its rotundity of form increases with the number of
large orifices, but in the adult stages the form varies, some being
conical, while others have the top divided into radiating ridges.

^E. officinalis^, variety ^dura^. Irregular, massive, horizontally
expanded, with conical process on upper surface.

GENUS ^Hippospongia^

^H. equina.^ Some of the sponges of this species are massive,
spherical, and attached by a small base; others are horizontally
expanded or cake-shaped; some have a depression in the upper
surface and become cup-shaped.

^H. equina^, variety ^cerebriformis^. Massive, circular,
cake-shaped, often depressed in the center, producing a cup-shape,
attached by broad base; surface broken up by parallel longitudinal
ridges having many tufts. Cup-shaped forms predominate, and have
a more or less rough surface. This is one of the species known as
grass-sponges.

^H. equina^, variety ^meandriformis^, the velvet sponge. The
surface of this variety has a protruding flattened cushion of fiber
which slightly resembles the convolutions of the brain-coral.
Sometimes these cushions are extended into long pencils. The oscula
are large and ragged on the edges; the shape is irregular. The
average size is seven to eight inches in diameter. (Plate XL.)

^H. equina^, variety ^elastica^ (variety ^agaricina^, Hyatt), the
yellow sponge. This is the second grade of commercial sponge,
corresponding to the Zimocca sponge of the Mediterranean. It is
found growing with the "sheepswool" in a depth of two to twenty
feet, and is abundant. It is massive and cake-shaped. The whole
surface is a network covered with numerous small, fine cones.
(Plate XL.)

The variety ^dura^ is classed with this species, which it resembles
in appearance, though it is harder in texture.

^H. canaliculata.^ Massive, frondose; more or less horizontally
expanded; bears finger-like processes of varying development on the
upper surface.

^H. canaliculata^, variety ^gossypina^, the sheepswool sponge. This
is the highest grade of the commercial bath-sponge. It is called
"sheepswool" because, perhaps, of its irregular shaggy surface. It
is covered with tufts, the larger oscula occupying the intervening
depressions.

Sometimes these are very numerous, the whole interior being
cavernous; again, the structure is more dense, with fewer large
openings and more of the small ones between the tufts; again, the
depressions are filled up so that the surface has fewer tufts.
When living, the color is shining black. This is the best sponge
found on the American coast, and although of coarser texture than
the best Mediterranean sponges, it is more durable and quite as
elastic. (Plate XL.)

^H. canaliculata^, variety ^flabellum^ (^Spongia graminea^, Hyatt).
This is one of the species of sponges of the third commercial
grade, which bear the trade-name of "grass-sponge." The shape is
cone-like, with either a flat or a funnel-shaped top. The oscula
are on the upper surface. The sides are fluted with deep furrows
which contain the small incurrent apertures.

GENUS ^Hircinia^

^H. campana.^ The normal variety is vase-shaped, but the species
varies greatly in form. Some varieties have branches. When living,
its color is black. It is found at Key West in four to forty feet
depth. (Plate XXXVIII.)

II

C[OE]LENTERATA
(POLYPS)

Phylum

=C[OE]LENTERATA=

Classes

=HYDROZOA=

(/Zooephytes, small Jellyfishes, and a few Corals/)

=SCYPHOZOA=

(/Large Jellyfishes/)

=ACTINOZOA=

(/Sea-anemones and most of the Stony Corals/)

=CTENOPHORA=

(/Comb-jellies/)

C[OE]LENTERATA

The animals included in the phylum =C[oe]lenterata= were once all called /zooephytes/, or animal plants, because of their resemblance to vegetable forms. The name /C[oe]lenterata/ is derived from two Greek words meaning "hollow" and "intestine," and it describes the anatomical structure of each member of the group. They are commonly known as polyps. In the simplest forms the parts which perform the different functions cannot be distinguished one from the other, and even in higher forms there is but little differentiation. Shakspere's description of old age applies to them: "Sans teeth, sans eyes, sans taste, sans everything."

Nevertheless, this very low order of animals has, like the higher orders, such a diversity of form and habit as to require classification. Some of them are stationary, and of these some branch like plants; some move about by the aid of tentacles, some move by means of vibrating cilia, and others move by the contraction and expansion of the soft body.

Cuvier included them in his /Radiata/, a class comprising all the animals whose parts diverge or radiate from a central axis. Recent classification has divided the radiate animals into several classes. This arrangement of parts is obviously quite different from that of bilateral symmetry, or the disposal of parts on each side of a longitudinal axis. The type of radiate structure is shown in polyps. The body is a sac, in the center of which is another sac or axis. This is the digestive cavity. Vertical partitions extend from the central to the outer sac, forming distinct [pg114] divisions or chambers. The number of divisions varies with the different species and also with the age of the animal. Other partitions start from the outer sac, and extend toward the central axis, but do not unite with it. These partitions, called /mesenteries/, are always in definite multiples, varying in different species, new divisions growing between the first partitions in regular order. On the inner edge of these partitions the eggs of the animal are formed, which, when mature, drop into the chambers and pass through openings into the inner sac, or digestive cavity, and out of the mouth into the water.

The animals are classed according as the eggs are formed on all or on special partitions, those being of the highest order where a limitation and constancy of function is maintained. The upper surface of the body has hollow tentacles, each one of which opens into one of the chambers and extends outward. All parts of the animal communicate, and whatever enters the mouth circulates through the whole structure; and when assimilation is completed the residue returns by the same road and is expelled through the mouth. This structure is common to all polyps; but there are great differences in their texture, some being soft and some horny, while others deposit a calcareous skeleton (corals). Some grow in colonies, like the hydroids and corals, and are stationary, others are free and independent; some have but few, others have many tentacles; and they differ widely in size, form, and color.

Hydroids, sea-anemones, corals, sea-fans, starfishes, and sea-urchins are different examples of the radiate structure. They are not, however, all of them polyps.

The /C[oe]lenterata/ are divided into four classes: =Hydrozoa=, which include the colonies of zooephytes which resemble seaweeds, the small jellyfishes which are born of these colonies, and the /millepores/, which are colonies of zooephytes which secrete a stony instead of a horny skeleton, yet differ in some respects from other [pg115] stony corals; =Scyphozoa=, large jellyfishes, many of which do not have the double form of development; =Actinozoa=, the sea-anemones and the different forms of stony corals, including sea-fans, sea-pens, etc.; =Ctenophora=, the jellyfishes commonly known as comb-jellies. [pg116]

TABLE SHOWING THE CLASSIFICATION OF THE HYDROZOA DESCRIBED IN THIS CHAPTER

Class =HYDROZOA=

Order =LEPTOLINIAE= (/Branched colonies or shrub-like communities of
hydroids; some permanently fixed, others liberate swimming-bells/)

Suborder =ANTHOMEDUSAE= (/Zooeids not covered by protective sheath;
umbrella of medusae strongly arched; egg-sacs in manubrium/)

Division TUBULARIANS (/Without polyp-cups/)

Genera and Species

^Clava leptostyla^
^Hydractinia polyclina^
^Coryne mirabilis^
^Sarsia^
^Eudendrium ramosum^
^Tubularia indivisa^
^T. Couthouyi^
^Parypha crocea^
^Corymorpha pendula^
^Hybocodon prolifer^
^Pennaria tiarella^
^P. gibbosa^
^Bougainvillea superciliaris^

Suborder =LEPTOMEDUSAE= (/Zooeids covered with cup-like sheaths;
umbrella of medusae thin and not very convex; egg-sacs in line
of radial canals/)

Division CAMPANULARIANS (/Polyp-cups stalked/)

Genera and Species

^Obelia commissuralis^
^Eucope diaphana^
^Oceania languida^
^Clytia poterium^
^C. bicophora^

Division SERTULARIANS (/Polyp-cups sessile/)

^Sertularia pumila^
^S. argentea^
^S. cupressina^

Division PLUMULARIANS (/Polyp-cups on one side of branches
only/)

^Aglaophemia struthioides^
^Plumularia falcata^

Family =GERYONOPSIDAE=

^Tima formosa^

Family =AEQUOREIDAE=

^Zygodactyla groenlandica^

Order =TRACHYLINAE= (/No fixed zooephyte stage; always
free-swimming medusae/)

Suborder =TRACHYMEDUSAE= (/Tentacles spring from margin of
umbrella; manubrium long; gonads in connection with radial
canals/)

Genera and Species

^Trachynema digitale^

Suborder =NARCOMEDUSAE= (/Tentacles spring from midway between
summit and margin of umbrella; manubrium short; gonads in
connection with manubrium/)

Order =HYDROCORALLINA= (/Skeleton of carbonate of lime/)

^Millepora alcicornis^

Order =SIPHONOPHORA= (/Pelagic free-swimming colonies; exhibit
extreme polymorphism of zooeids/)

^Nanomia cara^
^Physalia arethusa^
^Vellela limbosa^

TERMS USED IN DESCRIBING HYDROIDS

^C[oe]nosarc^ ("common flesh"): The fleshy axis, or organized living bond, by which the zooeids are organically united to one another. It consists of three layers: ectoderm, endoderm, and mesoderm.

^Ectoderm^ ("outside skin"): The outside one of the three organized layers of which every hydroid is composed.

^Endoderm^ ("inside skin"): The innermost layer.

^Gonangium^ ("seed-vessel"): The external horny receptacle within which the gonophores are developed.

^Gonophore^ ("seed-bearing"): A generative zooeid.

^Hydranth^ ("water flower"): A nutritive zooeid.

^Hydrorhiza^ ("water-root"): The part of the colony which fixes it to other bodies, like a root.

^Hydrosoma^ ("water-body"): The entire hydroid colony.

^Hydrotheca^ ("water-receptacle"): The cup-like, horny receptacle which protects the hydranth.

^Mesoderm^ ("middle skin"): A layer which lies between the ectoderm and the endoderm.

^Nematophore^ ("thread-bearing"): The name of peculiar bodies developed in certain genera from definite points; characteristic of plumularians.

^Perisarc^ ("around flesh"): The transparent, chitinous shell, or unorganized outer membrane of horny consistency, which covers to a greater or less extent the soft parts of the colony.

^Zooeid^ ("animal form"): One of the animals which form the colony. [pg119]

CLASS =HYDROZOA=

SEAWEED-LIKE ZOOePHYTES AND SMALL JELLYFISHES

The hydroids have been called the nurses of jellyfishes. From casual observation these two forms would not be associated together, for the shrub-like organisms, which so much resemble plants that they are often collected and preserved as seaweeds, suggest only vegetable life. Examined with a glass, however, they disclose their animal nature. Along the stems, arranged in various ways, are small cups, from which protrude the numerous moving tentacles of the little polyps living within them.

/Hydroids are colonies of associated animals living a communal life./ The multitude of individuals composing the colony are invested with a horny covering, the /perisarc/, which in some genera assumes a tree-like form. Through these stems and branches runs a fleshy tube, a thread of animal substance, which connects in one living whole the zooeids, or individuals of the community. There is division of labor, as in other communities: some of the zooeids obtain the food for the colony, and have tentacles around their open mouths; others have no mouths, but reproduce the species, and at certain stages of development liberate swimming-bells, or small jellyfishes (medusae).

The typical hydroid colony is attached by a kind of creeping stem from which arises a vertical axis, which gives off short lateral, alternate branches bearing zooeids at their ends. There is often more complex branching. The zooeids in certain genera (tubularians) are uncovered; in others (sertularians) they are incased in a glassy, cup-like, horny sheath.

Three kinds of zooeids, polyps, or hydranths--as they are indiscriminately called--are attached to the stem. Those having an [pg120] open end and a crown of tentacles are the nutritive individuals. Small, club-like dilations are immature zooeids. The /blastostyles/, or reproductive zooeids, are long, cylindrical, mouthless, and covered. At maturity the cover is ruptured, and the medusae have the appearance of a pile of thin saucers attached by the middle of the convex side. When at length these saucers are set free as little medusae, or jellyfishes, the convex side of each saucer, or swimming-bell, is called the /ex-umbrella/; the concave, under side, the /subumbrella/. From the center of the subumbrella projects the /manubrium/, or stomach of the animal. At the free end of the manubrium is a four-cornered mouth. From the attached end of the /manubrium/ four tubes or canals diverge, and, extending through the animal, open into a circular canal which runs around the margin of the umbrella. When the medusa is as above described, it has reached the highest point in its development.

When the medusa has matured, it lays eggs, known as /planulae/. These are spherical bodies covered with cilia (hairs), by means of which they swim about for a time; but they finally attach themselves to some object, there to grow and develop into hydroid colonies. The cycle of life is thus completed. This process is known as /alternation of generation/, or /metagenesis/, one life-history containing two quite different forms of being. The term of life of an individual is one year, the zooephyte stage beginning in the autumn and the medusa stage in the spring.

Some medusae, besides reproducing by means of eggs, multiply by budding, small medusae growing on the manubrium or on the margin of the umbrella. /Sarsia/ and /Lizzia/ sometimes increase by budding.

The /Hydrozoa/ are not all of the above type. In the sertularians the zooeids perish on the stem and have no medusa life, their reproductive element giving rise to the hydroid form without metamorphosis. The /Trachylinae/ have no hydroid life, being always free-swimming medusae; others, the /Siphonophora/, live a hydroid life which is unattached, the colony floating on the ocean; the millepores secrete calcareous skeletons and always remain fixed, reproducing by budding. [pg121]

Hydroids are very abundant, but are comprised in the few groups mentioned: namely, those which live only in the fixed colonial state; those which have alternation of generation, being first hydroids and then swimming-bells, or medusae; those which live always in the medusa state, the eggs of the jellyfish developing at once into other medusae; and the /Siphonophora/, or those which have a floating colonial state, the hydroid never being attached, but floating at large and capable of locomotion, some of the colony having the function of propulsion.

Hydroids are particularly interesting as exemplifying the close resemblance that may exist in outward appearance between animal and vegetable life and as illustrations of communal life and of the alternation of generation. A few examples of different types are given below.

ORDER =LEPTOLINIAE=

The members of this order agree in all essential particulars being branched colonies having two principal forms of zooeids, the nutritive and the reproductive. Some genera attain the length of several inches, or even feet; others are very small tufts growing on shells and seaweeds. The cup may completely inclose the zooeid and be close to the stem (sessile), as in sertularians; it may be on the end of a short stalk, as in campanularians; or it may not reach above the base of the zooeid, as in tubularians. The genera are based upon these differences in the perisarc.

The hydroids, like all other classes, exist in such great variety that it would be impracticable to describe here the many named species; but to recognize the genera is simple. A long tubular pedicel without a cup is characteristic of the tubularians; the campanularians have an arborescent form and bell-shaped cups on stalks; the sertularians have sessile cups; and the plumularians have a feather-like form, with zooeid-cups on one side only of the branches.

The beautiful and varied structure of these "animal plants" is most interesting, and to be fully appreciated they should be seen [pg122] in life and examined with a glass. Some species are confined to deep water, but many are littoral and to be found in tide-pools, in the chinks and crannies of rocks, under stones, and under the hanging /Fucus/. The horny skeletons of large varieties are frequently washed ashore, and in their tangled masses smaller living species often may be found.

THE TUBULARIANS

This division is characterized by zooeids borne on long, slender stems which are sometimes simple and small, sometimes branching and eight to ten inches long. The zooeid has two rows of tentacles, the central one being sometimes on a kind of proboscis. The reproductive zooeids are in bunches, sometimes below the outer row of tentacles, sometimes between the two rows. The perisarc does not cover the zooeid. In color they are commonly red or yellow.

GENUS ^Clava^

^C. leptostyla.^ This species is found growing on /Fucus/, on the
under side of stones at low-water mark, and in tide-pools, where
it often covers several feet of the surface of the rock with a
delicate velvet-like carpet. It is red in color and is, apparently,
a soft and tender species, but it thrives on the most exposed
beaches. The colonies are cylindrical tubes about one quarter of an
inch in height, rising from a creeping stem (/hydrorhiza/). Each
tube is surmounted by a zooeid with fifteen to thirty tentacles,
which is constantly changing form by its contractions. Below the
tentacles are reproductive buds arranged in clusters. Common from
Long Island Sound northward. (Plate XLI.)

GENUS ^Hydractinia^

^H. polyclina.^ The soft, pinkish covering often seen on shells
inhabited by hermit-crabs. This association of two different
kinds of animals is known as /commensalism/, and is a partnership
formed for the benefit of one or both the individuals. In this
case the mossy appearance of the hydroid conceals the shell,
while the stinging-cells with which it is invested are weapons of
defense against the enemies of the crab and also help to paralyze
its prey. In return for these favors the colony is moved about,
thereby obtaining perhaps better oxygenation. Originally it was
thought that /Hydractinia/ lived only on the shells occupied by
hermit-crabs, and that the nomadic life was essential to its
existence; but this is not the case, for it is also found growing
on rocks in tide-pools. These colonies arise from a creeping stem,
which forms a horny, root-like [pg123] network over a surface
and develops at intervals projecting points on which the zooeids
live. Each colony consists of feeding members, of reproductive
members, and of a third kind which seems to have a protective
function. These last are more slender than the others, and are
without tentacles, but are armed with lasso- or stinging-cells. The
colonies are of different sexes, the male being lighter in color
than the female colonies. The eggs develop into planulae, which swim
about for a while and then give rise to other colonies. It is found
from New Jersey northward, and is very abundant in Long Island
Sound.

GENUS ^Coryne^

^C. mirabilis.^ A hydroid about one inch high, growing in patches
and appearing like tufts of moss on rocks between tide-marks.
When highly magnified it shows club-shaped tubes with pedicels,
terminating in zooeids, scattered over the swollen ends. The
medusa-bud is larger than the others and is lower on the tube. It
liberates a swimming-bell, which is called /Sarsia/. (Plate XLI.)

^Sarsia^

^S. mirabilis.^ This medusa of /Coryne/ is from one quarter to
three quarters of an inch in diameter when full-grown. Its umbrella
is nearly hemispherical, and from the center hangs a manubrium.
From the margin of the umbrella hang four very long tentacles. The
shape of its body and the length of its tentacles and proboscis are
constantly changing as it moves in the water. These little medusae
are very plentiful in the spring and summer, and swim rapidly in
all directions near the surface of the water.

GENUS ^Tubularia^

^T. Couthouyi.^ This species is found in the same places as
/Parypha crocea/. The stem is three to six inches long, and is
inclosed in a horny sheath, which is more or less ringed or
jointed, or it may be smooth throughout. The head, when the
tentacles are expanded, measures one and a half inches in diameter.
It has a proboscis covered with tentacles, disposed in series,
which grow successively shorter, the last being merely papillae. The
medusa-buds hang in clusters between the outer tentacles and the
proboscis. The animal grows in bunches of five to ten tubes, which
spring from a creeping, tangled stem. (Plate XLI.)

GENUS ^Parypha^

^P. crocea.^ This is one of the most beautiful of the tubularians.
It has a large, drooping head on a stem three to [pg124] four
inches long. It is bright red in color, and from the center of the
circle of tentacles the reproductive zooeids hang in a cluster, like
a bunch of grapes. It does not liberate swimming-bells. It is found
in bunches on piles of wharves and bridges, in brackish water, on
the eastern coast as far south as Charleston, South Carolina.

^Corymorpha pendula.^ See Plate XLII.

GENUS ^Hybocodon^

^H. prolifer.^ One of the largest tubularians, somewhat resembling
/Parypha crocea/. It is deep orange in color, and the head is
erect on a long stem. The reproductive zooeids are in a cluster in
the center of a double row of tentacles, and resemble a basket of
fruit. It grows singly, or in groups of two or three, in shaded
tide-pools, which are protected from the surf, and in which the
water is very pure. It is found on the Massachusetts coast, but is
not common. This species liberates swimming-bells. (Plate XLI.)

GENUS ^Pennaria^

^P. tiarella.^ The branches are arranged alternately and at right
angles to a central stem or axis; they taper, being shortest at
the top and bottom of the stem. The zooeids are red in color, and
are arranged along the upper side of branches at considerable
intervals. The stems are black and beaded, being constricted at
intervals. The zooeids have two rows of tentacles, the upper ones
on a small proboscis. From the lower part of the proboscis deep,
bell-shaped bodies, which eventually become swimming-bells, are
developed. The species is found on rocks and eel-grass along the
whole eastern coast.

^P. gibbosa.^ A species similar to /P. tiarella/, found on the
coast of Florida.

GENUS ^Bougainvillea^

^B. superciliaris.^ This hydroid is found in tide-pools on the
New England coast, growing in clusters, about two inches high,
attached to rocks or to mussel-shells. The stem is very slender,
and branches. It is red in color. The medusae which it liberates
are found in great numbers in the spring. The tiny swimming-bells
are nearly globular. The tentacles are long, are arranged in
four clusters on the margin, and extend in every direction. The
manubrium is yellow and short, and the mouth is concealed by four
clusters of short tentacles. On these oral tentacles [pg125]
the eggs of the animal are produced. In its habits it is sluggish,
often remaining in one position for several days.

THE CAMPANULARIANS

The hydroids which have an open, bell-shaped cup at the termination of a short, stalk-like stem, or branchlet, are mostly campanularians. This division embraces jellyfishes of different families. Many medusae cannot be referred with certainty to the hydroids from which they sprang, and the medusa-buds of many of the hydroids have not been noted.

GENUS ^Obelia^ (Plate XLII, A)

^O. commissuralis.^ This is a delicate, much-branched hydroid, five
to six inches long, found at low-water mark in tide-pools, attached
to stones and seaweeds, along the rocky shores from Nova Scotia
to South Carolina. Its branches are arranged spirally and spread
nearly at right angles to the main stem, and the main branches
subdivide in a similar manner. Every interval of the stem has a
slight curve, and at the base of every branch there are four or
five rings. The ultimate branches, or pedicels, bear at their ends
bell-shaped cups which have even edges, but are twelve-sided and
slightly incurved. The pedicels are ringed for the whole length.
The reproductive cups on short ringed pedicels are larger than
the others, and occupy the angles of the branches. These cups are
constricted and again expanded at the apex, forming an urn-like top.

GENUS ^Eucope^

^E. diaphana.^ This species is often abundant on the fronds of
/Laminaria/ washed ashore, and also on /Rhodymenia/ and /Fucus/.
It has a creeping base, zigzag in form, but keeping a straight
course, and in its branching often forming a network over the
surface of the flat fronds. At each angle of the creeping stems
rises a pedicel about an inch high, which inclines in the direction
of the stem and terminates in a zooeid-cup similar in form to that
of /Obelia/. The medusa which this hydroid liberates is called
/Thaumatias diaphana/. The swimming-bell is very shallow and thin,
turning inside out at almost every pulsation. The tentacles are
numerous and rigid like stiff hairs. This little medusa is very
active and is abundant. The species is found from Long Island Sound
northward.

GENUS ^Oceania^

^O. languida.^ This medusa is one inch in diameter and one half of
an inch high, and is so delicate and transparent that it is hardly
visible except in its outlines. In its early stages it is nearly
spherical and has no tentacles; later the disk flattens and has
from thirty-two to thirty-six [pg126] tentacles and numerous
eye-spots. When disturbed it flattens its disk and folds together,
leaving its tentacles sprawled in every direction. It is very
languid in its movements, and often remains in one position for
hours. These medusae are found only in the hottest hours of the day,
but are very plentiful then, shoals of them often stretching for
miles, and so thick as to touch one another. Their habitat is the
New England coast.

GENUS ^Clytia^

^C. poterium.^ This hydroid is found creeping over seaweeds in
tide-pools from Long Island Sound northward. The main stem is
prostrate, or root-like, running over the body to which it is
attached. The stems rise as do the tubularians. The sterile zooeids
are on single stems about one quarter of an inch high. The stems
are faintly ringed for their entire length, and at the top have
a distinct ring, on which rests an open, bell-shaped cup, which
is smooth around the rim. The reproductive zooeids are on very
short pedicels, and the cups are long and cylindrical, with a wavy
outline. (Plate XLII, A.)

^C. bicophora.^ This species is found in the same places as the
preceding, and is of about the same size. The long stems are more
or less ringed and sometimes branched. The edges of the cups are
notched. The medusa-buds are urn-shaped and ringed, and are on very
short pedicels. (Plate XLII.)

THE SERTULARIANS

The sertularians are distinguished by the horny cup, which is sessile--that is, set directly against the stem instead of being raised upon a stalk. They are among the most common objects of the beach, and, like the plumularians, are often mistaken for plants by the amateur collector and are gathered and pressed as seaweeds. They are found everywhere along the coast. They zigzag over the fronds of seaweeds or hang in fringes upon them, as well as upon rocks, stones, and shells. They well repay close examination with a glass. Every open cup bears a wreath of tentacles, which makes the branch a spray of stars. This is not an inappropriate comparison, for besides their starry shape some species emit a phosphorescent light.

GENUS ^Sertularia^

^S. pumila.^ The most abundant of all the hydroids on the northeast
coast is this species, which is found in profusion upon /Fucus/
and other seaweeds, and mingled with them upon the rocks. It
is easily distinguished from the campanularians because its
zooeid-cups are close against the stems (sessile) instead of on
stalks or pedicels. The stem creeps [pg127] over the fronds
of seaweeds, often crossing and recrossing in a tangled mass. At
short intervals the upright, straight branches rise to one inch or
one and a half inches in height, and are more or less branched.
All except the creeping stems are close set on each side, with
cylindrical zooeid-cups which turn outward at the ends. The cups of
the reproductive zooeids are not sessile; they are much larger than
those of the nutritive ones and are urn-shaped. (Plate XLII, A.)

^S. argentea.^ This is a beautiful species, common from New Jersey
northward. It has a profusion of silvery branches on a dark stem.
The colonies are often a foot or more long, and the branches at
the top and bottom of the stem are shorter and fewer than those in
the middle of the colony. The zooeid-cups are nearly cylindrical,
pressed closely to the stem, nearly opposite or subalternate to one
another, and end in pointed tips. The medusa-bud is urn-shaped,
with two horns at the top. (Plate XLII, B.)

^S. cupressina^, the sea-cypress. This species is similar to
/S. argentea/, but the main stem is thicker and longer, and the
branches less crowded and less subdivided. The branches are arched
or drooping, instead of straight, and gradually decrease in length
at some distance from the lower and upper parts of the stem,
giving a spire-like apex, the stem often continuing into a bare,
branchless extremity. The zooeid-cups are tubular, not much narrowed
or divergent above, and two-lipped on the margin. It is found from
New Jersey northward. (Plate XLII, B.)

THE PLUMULARIANS

These hydroids are feather-like in the manner of branching, short lateral branches being arranged on each side of a long central stem. In some species the stems are naked below and resemble quills. The zooeid-cups are only on one side of the short branches. (Plate XLIII.)

GENUS ^Aglaophemia^

^A. struthioides^, the ostrich-plume. This species, which is
found on the Pacific coast, is perhaps the most beautiful of the
hydroids. It varies in size and color, but always suggests a small
ostrich-plume. The zooeid-cups are arranged in a single row on one
side of each short branch, and the main stem has a joint between
each of the branches, which are placed quite close together. The
rims of the cups have sharp-pointed teeth, and from the top emerge
three tubular projections, which are called /nematophores/, and are
supposed to be degenerate zooeids. At intervals a branch is replaced
by a cylindrical body covered with nematophores, and in these the
generative zooeids are developed. (Plate XLIII.)

GENUS ^Plumularia^

^P. falcata^ (Johnston), or ^Hydrallmania falcata^ (Hincks). This
species is found on shells and rocks near low-water mark from Long
[pg128] Island Sound northward. It is from four to twelve inches
high. The main stem is in long spiral turns, and at intervals has
spreading plumose branches. The zooeid-cups are tubular and closely
pressed against one another, and are ranged in rows on one side of
the branchlets; the apertures of the cups are plain and oblique.
(Plate XLIII.)

FAMILY ~GERYONOPSIDAE~

GENUS ^Tima^

^T. formosa.^ A very delicate and transparent medusa; size one to
two inches in diameter; bell conical; radial tubes four in number;
manubrium long, hanging far below the disk; four frilled appendages
diverging from the corners of the mouth; tentacles thirty-two;
egg-sacs white and following the line of the radial tubes in
undulating folds. This species is not very common; it is found on
the New England coast.

FAMILY ~AEQUOREIDAE~

GENUS ^Zygodactyla^

^Z. groenlandica.^ Medusa seven to eleven inches in diameter; disk
violet-colored and transparent; margin fringed with long, fine,
contractile tentacles of a darker violet color; numerous radiating
tubes; egg-sacs in slightly waved plates; manubrium hanging below
the line of the disk and with a thin frilled membrane depending
from it. Found north of Cape Cod in July.

ORDER =TRACHYLINAE=

The /Trachymedusae/ are characterized by their direct development, the egg of the jellyfish producing a medusa and not a hydroid colony.

GENUS ^Trachynema^

^T. digitale.^ Size one inch to one and a half inches in height;
rose-colored; the bell thin and hard, and conical at the top. The
swimming is effected by contractions of the muscular velum (the
band around the inner margin of the umbrella) instead of wholly by
the bell. The tentacles are long and numerous, and are curled up
when moving. The manubrium is long and has four expansions at the
mouth. Eight egg-cases [pg129] hang in long pendent sacs from the
upper part of the radial canals and reach nearly to the velum. Four
garnet-colored eyes in club-shaped processes are prominent on the
margin. The animal moves by jerks in straight lines.

ORDER =HYDROCORALLINA=

CALCAREOUS HYDROIDS

The genus ^Millepora^ ("thousand pores"), which is the type of this order, is a colony of animals, like other hydroids, which secrete calcareous instead of horny coverings. It differs from true corals in that the members of a colony perform different functions, whereas in true corals each member of a community is a complete individual. It differs also in the arrangement of the stony partitions, which in =Hydrocorallina= are the outside coverings and connecting canals, but in true corals are vertical partitions inside the animal, between the inner and outer sacs, as explained on page 114.

GENUS ^Millepora^

^M. alcicornis^, elk-horn coral. This beautiful coral, which is
abundant in Florida and contributes to the building of the reefs,
rises in broad expansions, more or less lobed, and suggests by its
shape the object for which it is named. The whole mass is porous,
being traversed by innumerable canals. Its surface, although
smooth compared with that of other corals, is covered with very
minute pores, which are of two sizes. The larger ones are the
/gastropores/, or stomach-pores, in which the nutritive animal
lives; it has a cylindrical body, with four knob-like tentacles and
a mouth. Placed more or less irregularly around the gastropores are
smaller pores, the /dactylopores/ (finger-pores), from which emerge
slender mouthless processes, or /dactylozooeids/, with tentacles and
stinging-cells. These seem to be the guard-polyps of the community.
The cups occupied by the zooeids are shallow. As one animal dies,
another succeeds it and builds a horizontal partition separating
the new cup from the old one. Thus the stony mass increases in size
by the progress of succeeding generations of zooeids. The living
animal occupies only the outer, open space. (Plate XLIV.) [pg130]

ORDER =SIPHONOPHORA=

FREE-SWIMMING COLONIES

This order of hydroids consists of free-floating communities and is one of peculiar interest, since it shows in a clear manner the special function of each individual member of the colony, and illustrates better than the foregoing, perhaps, the curious forms of animal life which this class presents.

GENUS ^Nanomia^

^N. cara.^ This species is found on the New England coast. The
members of the community are arranged along a hollow stem about
three inches long which opens into every individual. At the top
of the stem is a sac, or float; just below this is a group of
swimming-bells which have no manubrium or mouth, and whose sole
function is to provide locomotion for the community; and below
these are three sets of zooeids, each having a triangular shield
and tentacles. The tentacles are longer than the main stem. One of
these last groups consists of the nutritive members, the mouths
of the community, resembling manubriums of swimming-bells out of
place. Each one has at the point of attachment a bunch of long,
delicate tentacles having pendent knobs of lasso-cells. A second
group, also with mouths, has shorter tentacles which are carried in
spiral coils. The members of the third group have but one tentacle
each and resemble the float at the end of the stem; presumably
these drop off and produce new colonies. There are also on the
lower part of the stem other reproductive members, which resemble
the clusters of buds seen on [pg131] tubularian hydroids. These
animals are pink in color and move through the water with a
graceful swaying motion.

GENUS ^Physalia^

^P. arethusa^, the Portuguese man-of-war. This colony is perhaps
the best-known one of the group, since it attracts much attention
in Southern waters, and is also one of the most remarkable
examples of an animal community. The most prominent part of the
compound body is the float, an oblong pear-shaped bag, full of
air, which floats on the surface of the water. Its color is bright
blue, varying to rose. On the upper side of this air-vessel is a
crest, or sail, and from the under side depend long tentacles, or
streamers. Some of these tentacles are covered with stinging- or
lasso-cells; some are the feeding zooeids, with flask-shaped
bodies, and some, which look like bunches of grapes, are the
reproductive zooeids. The tentacles in this curious cluster are
all close together and hang from one side of the float, near the
broader end. The longest are on the outside, which may be called
the windward side, since they serve to keep the crest, or sail,
before the wind; and when the wind is strong they stretch out to
a remarkable length,--forty to fifty feet,--acting as anchors to
keep the colony from being driven ashore. They also change its
course by raising the pointed end of the float, thus forcing it to
"come about." These long tentacles, ordinarily carried more or less
curled up, are in bunches of two to four, and emerge from a common
stem. Clusters of similar, but smaller, tentacles alternate with
the larger ones, but grow somewhat nearer the pointed end of the
float; these are purely locomotive organs. Next come two smaller
sets of appendages, also of unequal size, which are the nutritive
organs of the community. They are clustered together on a stem like
the others. The appendages of the third kind are small, resemble
bunches of grapes, and are scattered among the nutritive hydrae.
These last are the reproductive zooeids of the community.

GENUS ^Vellela^

^V. limbosa.^ This hydroid is abundant on the Florida coast. It
has a bright-blue, flattened, oblong, bladder-like float, four to
five inches [pg132] long, which, is divided into a number of
concentric, communicating compartments. The margin of the float is
entire, and a triangular sail extends diagonally across the top. On
the under side is a single mouth on a manubrium, and surrounding it
are a large number of short thread-like appendages having different
functions. Some of them are feelers, others bear reproductive buds,
and others have stinging-cells. Associated with /Vellela/ is an
allied hydroid called /Porpita/, which has no sail, and in company
with these two is a jellyfish called /Rataria/, which is supposed
to be the offspring of one or the other of them.

TABLE SHOWING THE CLASSIFICATION OF THE SCYPHOZOA DESCRIBED IN THIS CHAPTER

Class =SCYPHOZOA= (/Velum usually absent and tentaculocysts
present; the eggs are discharged into a gastric cavity. The type
of this class is Aurelia/)

Order =STAUROMEDUSAE= (/Umbrella conical or vase-shaped; no
tentaculocysts/)

Genera Species

^Lucernaria^ ^L. auricula^

Order =PEROMEDUSAE= (/Umbrella conical and divided by transverse
constrictions; four interradial tentaculocysts/)

^Pericolpa^ ^P. quadrigata^

Order =CUBOMEDUSAE= (/Umbrella four-sided, cup-shaped; four
perradial tentaculocysts/)

^Charybdaea^ ^C. marsupialis^

Order =DISCOMEDUSAE= (/Flattened, saucer-like or disk-shaped
umbrella; radial tubes branched; eight tentaculocysts/)

Suborder ~CONNOSTOMAE~ (/Very small; marginal tentacles; short
and solid; mouth square without arms/)

Suborder ~SEMOSTOMAE~ (/Square mouth with four long arms;
tentacles long and hollow/)

^Aurelia^ ^A. flavidula^
^Cyanea^ ^C. arctica^
^C. fulva^
^C. versicolor^
^Linerges^ ^L. mercurius^
^Pelagia^ ^P. cyanella^

Suborder ~RHIZOSTOMAE~ ("/Root-mouth/") (/Mouth obliterated by
growth of oral arms across it; tentacles absent/)

^Cassiopeia^ ^C. frondosa^

[pg134]

CLASS =SCYPHOZOA=

THE LARGE JELLYFISHES

There is perhaps no marine animal which excites more wonder than the jellyfish. Its transparency, its graceful rhythmical movements, its long streaming tentacles, the variety and eccentricity of its form, and often of its color, attract attention, and one naturally desires to know something of its life-history. Jellyfishes are also called /medusae/, because their long appendages suggest the locks of the Gorgon; /acalephs/, on account of their stinging or nettle-like properties; and sun-jellies, sea-blubbers, etc., because they float upon the surface during the warmest part of the day, when the sun is high. The name /jellyfish/ is inappropriate, since the animal in no way resembles a fish except in the fact that it swims; but it is, nevertheless, the commonest name.

Jellyfishes vary in size from that of a pinhead to six or seven feet in diameter. They differ in the number, size, and position of the tentacles, the number of the radial canals, the form of the manubrium, the position of the egg-sacs, etc.; but the general plan of the internal structure is the same in all species. In shape they are compared to a mushroom. From the center of an umbrella-like top falls a central organ like the stalk of a mushroom. It is called the /manubrium/ and is the mouth and stomach of the animal.

From the top of the manubrium radiate straight or branched tubes, which are connected with a canal which runs around the whole margin of the umbrella. Extending around the inner circumference of the disk in certain species (usually the hydroid medusae), there is a horizontal shelf, called the /velum/, or veil, because it sometimes falls like a veil. [pg135]