Chapter XVII: Introduction: Classification—hydrozoa—eleutheroblastea—milleporina (7)

They differ from all the Coelenterata in the absence of nematocysts, and in the presence in development of a definite mesoblast. The character from which they derive their name, Ctenophora, is the presence on the surface of bands of swimming plates. The plates are called the "combs" (κτείς, gen. κτενός = a comb) or "ctenophoral plates." They occur in all genera included in the Phylum except in _Coeloplana_ (Fig. 183, p. 422).

Another peculiarity of all Ctenophora (except the Beroidae) is the presence, at some stage in the life-history, of two long and extremely contractile tentacles. There is also a well-developed sense-organ (statocyst) in the centre of the aboral area of the body.

The Ctenophora differ from the Turbellaria in the presence of the combs and of the two long tentacles, in the position and relative importance of the statocyst, and, with the exception of _Coeloplana_, in the general characters of the alimentary canal.

SHAPE.—Several of the Ctenophora are conical or spherical in shape, but exhibit at the pole where the mouth is situated {413}(Fig. 180, _M_) a slight conical projection, and at the opposite pole where the sense-organ is placed a slight depression (_Ab_). In others, the sides of the body are drawn out into a pair of wing-like lobes (Lobata), and the body is considerably flattened or compressed (Fig. 181). The Cestoidea have a long flattened ribbon- or band-shape (Fig. 182), and the Platyctenea (Fig. 183) are flattened in the oro-apical axis and exhibit a well-marked distinction between the dorsal and ventral surfaces. The shape of _Beroe_ is that of a hollow cone or thimble.

CTENOPHORAL PLATES.—In many Ctenophora eight lines can be traced, like the lines of longitude on a globe, from the area of the sense-organ to the base of the mouth-cone or hypostome. In the course of these lines are situated the ctenophoral plates. In some species they extend along the greater part of these lines of longitude, but in others they are more restricted. That part of the line that bears the plates is called the "rib" or "costa." These plates or combs form the principal organs of locomotion of the Ctenophores. They consist of a row of cilia fused at the base (cf. p. 141) to form the plate, but free at the extremity where they form the comb-like edge. They are alternately raised, by a rapid contractile action, and then slowly flattened down again. The plates are raised in succession from the aboral to the oral end of each rib, and the appearance given to the bands in the living animal is that of a series of waves travelling down the lines of longitude from the sensory area towards the mouth. The effect of these rhythmic movements of the combs is to {414}drive the animal slowly through the water with the oral cone forwards. In some Ctenophores the costæ are phosphorescent.[427]

TENTACLES.—In all the Ctenophora, except the Beroidae and the adult stages of Lobata and Cestoidea, there is a single pair of tentacles. They are attached to the base of a blind funnel-shaped pit which opens to the exterior near the equator of the animal's body. The pits are on opposite sides of the body, and the plane which passes through them both vertically divides the body into approximately equal parts. It is called the "tentacular" or "transverse" plane (Fig. 180). The plane at right angles to this, which also passes through the mouth and statocyst, is called the "sagittal" plane.

The tentacles are solid, and in the Cydippidae, of considerable length. During life they are usually extended, and trail behind the animal as it progresses through the water. But they are extremely contractile, and when the animal is alarmed are suddenly withdrawn into the shelter of the tentacular pits. Each tentacle usually bears a row of short pinnae. The surfaces of the tentacles and of their pinnae are crowded with remarkable cells which carry little globules of an adhesive secretion, and are called the glue-cells or "colloblasts." These cells stick to any foreign body they touch, and may be drawn out some distance from the tentacle, but they remain attached to it by a long spiral thread which unwinds as the cell is pulled out. Although the colloblasts have the function of catching prey similar to that of the nematocysts of Coelenterata, they are true animal cells and are not therefore homologous with nematocysts, which are the cell products of the cnidoblasts.[428]

The Lobata and Cestoidea pass through a stage in development called the Cydippiform or _Mertensia_ stage, when they possess a single pair of long tentacles similar to those described above. In the adult condition, however, these tentacles are absent, and their functions are performed by numerous small accessory tentacles or tentilla arranged in rows on definite lines along the body-wall.

SENSE-ORGAN.—At the aboral pole of the Ctenophore there is a hard granulated calcareous body, the "statolith." This is {415}supported by four tufts of fused cilia, and is usually covered by a dome of delicate protoplasmic texture, which is believed to be formed by a fusion of cilia. The dome enclosing the statolith is called the "statocyst."

Supporting the statocyst there is a circular or oval area of ciliated epithelium which is usually supposed, but on insufficient evidence, to be specially sensory in function. Extending from this area in the sagittal plane there are two strips of ciliated epithelium called the "polar fields."

The aboral sense-organ of the Ctenophora is one of the most characteristic organs of the Phylum. The aboral pole of the Medusae of Coelenterata is usually devoid of any special modification of the ectoderm of the bell, and in the Tiarid genus _Stomatoca_ the little tassel at the aboral pole of the Medusa cannot in any sense be regarded as a homologue of the sense-organ of the Ctenophore. If the aboral sense-organ of the Ctenophora can be compared with that of any other group of animals, it would be with the statocyst of many of the Turbellaria, such as that of _Convoluta_, but it is far more satisfactory to regard it as an organ peculiar to the Ctenophora and as having no true relationship with any sense-organ found in other animals.

ALIMENTARY CANAL.—The mouth of the Cydippiform Ctenophores opens into a sac-like chamber called the "stomodaeum," flattened in the sagittal plane and stretching from the oral pole as far as the centre of the body. The stomodaeum passes into a chamber flattened in the transverse plane called the "infundibulum." From the infundibulum a narrow tube passes in the direction of the aboral pole called the "intestine," and from the extremity of this four short tubes pass to the sides of the polar fields at the place where these fields join the sensory area. Two, or, in some cases, all four of these tubes open to the exterior; but they do not appear to serve the purpose of ejecting the undigested portions of the food, which usually pass to the exterior by the mouth as in Coelenterata and Turbellaria.

From the lateral extremities of the infundibulum four pairs of tubes pass to the equatorial region of the body, where each one joins a longitudinal vessel which runs immediately beneath the epithelium supporting the ribs. These are called the longitudinal or "sub-costal" canals. From the infundibulum there also {416}passes a single pair of blind canals, the "paragastric canals," one on each side of the stomodaeum, to end in the oral cone.

In the Lobata the paragastric canals communicate with the longitudinal canals under the transverse costae,[429] and send long blind processes into the lobes. In the Cestoidea the arrangement of the canals is considerably modified in adaptation to the needs of the ribbon-like body. In the Beroidae the paragastric and longitudinal canals are in communication by a peripheral network of canals, and in the Platyctenea there is also a network of canals but without any definite longitudinal vessels.

SEXUAL ORGANS.—Most of the Ctenophora are undoubtedly hermaphrodite, but Willey was unable to find ova in some of his specimens of _Ctenoplana_ that were producing spermatozoa. In the Cydippidea the ova are produced on one side of the longitudinal canal and the spermatozoa on the other. Each longitudinal canal therefore performs the functions of a hermaphrodite gland. When the sexual cells are ripe they escape into the infundibulum and are discharged by the mouth. In _Ctenoplana_ there are definite and direct male genital ducts.

The ova are very small when discharged and undergo complete segmentation in the sea water. The development of the Cydippidea is really direct, but there is a stage passed through in which the tentacles are relatively very prominent and situated close to the aboral pole, and this stage is very different in appearance from the adult. In the Lobata and Cestoidea there is, however, a definite larval stage, of the general appearance of a _Mertensia_, and during this stage fertile eggs and spermatozoa are formed and set free.

DISTRIBUTION.—Ctenophora are found at the surface of nearly all seas, and many of the genera have a cosmopolitan distribution. Some of the Lobata, the Cestoidea, and the Platyctenea are more commonly found in the warmer regions of the world. _Pleurobrachia pileus_, _Bolina infundibulum_, _Beroe ovata_, and _B. cucumis_ occur off the British coast.

Most of the Ctenophora are from 5 to 20 mm. in diameter, but _Beroe_ reaches the length of 90 mm., _Eucharis multicornis_ {417}a height of 250 mm., and _Cestus veneris_ has been found no less than 1½ metres from one extremity to the other.

Ctenophores usually go about in shoals, and in the case of _Beroe cucumis_ and _Eucharis multicornis_ the shoals may be of very great extent. _Pleurobrachia pileus_ of the British coasts is often found at the end of the season (July) as a series of isolated individuals; but in June they occur in small shoals, swimming so close together that they will choke a tow-net in a very short space of time.

CLASS I. TENTACULATA

Ctenophora provided with a pair of tentacles in the larval stages only or in both larval and adult stages.

ORDER I. CYDIPPIDEA.

This order includes a number of spherical or oval Ctenophores, with a pair of tentacles retractile into deep tentacular pits in the adult stage.

FAM. 1. MERTENSIIDAE.—The body is compressed in the transverse plane, and the ribs on the transverse areas are longer than those on the sagittal areas. The family includes the genus _Euchlora_, which occurs in the Mediterranean and in the northern part of the Atlantic Ocean. In _Charistephane_ there are only two enormous ctenophoral plates in each of the longitudinal tracts. These plates are so broad that they almost meet laterally to form two continuous circlets round the body of the animal. This genus is found in the Mediterranean, but a few specimens have also been obtained in the Atlantic.

In _Tinerfe_ the body is almost cylindrical, and there is a pair of kidney-shaped swellings at the sides of the aboral pole. It has a pale blue colour, and is found in the Guinea and south equatorial currents of the Atlantic Ocean.

The name _Mertensia_ has been given to several forms that are undoubtedly the young stages of genera belonging to the Lobata, but Chun retains the name _M. ovum_ for a species which is very abundant in the Arctic currents of the North Atlantic.

FAM. 2. CALLIANIRIDAE.—Two or four wing-like processes, into {418}which the longitudinal canals extend, are found at the aboral pole. _Callianira_ has two of these processes arranged in the transverse plane, and _Lophoctenia_ has four. _Callianira_ is found in the Mediterranean and in the Atlantic from the Arctic to the Antarctic waters.

FAM. 3. PLEUROBRACHIIDAE.—The body is almost spherical in form, and the eight ribs are equal in length.

This family includes the genus _Pleurobrachia_, in which the ribs extend for a considerable distance along the lines of longitude of the spherical body, but do not reach either the oral or the aboral areas. _P. pileus_ is the commonest British Ctenophore, and may be found in shoals in May, June, and July at the surface of the sea or cast up on the sand as the tide ebbs. It is widely distributed in the North Atlantic waters. _P. rhodopis_ of the Mediterranean has rather shorter ribs than _P. pileus_. Two new species have recently been described from the Malay Archipelago.[430] _Hormiphora_ (Fig. 180, p. 413) differs from _Pleurobrachia_ in having much shorter ribs, and in possessing two kinds of pinnae on the tentacles, those of the ordinary kind and others much larger and sometimes palmate in character. This genus has a world-wide distribution.

In _Lampetia_ and _Euplokamis_ the body is more cylindrical in shape than it is in the other genera, but the ribs and subjacent longitudinal canals extend up to the margin of the aboral field. Both these genera occur in the Mediterranean, but _Lampetia_ is also found in the Malay Archipelago.

ORDER II. LOBATA.

The body is considerably flattened in the transverse plane, and the sagittal areas are extended into the form of two wide peristomial lobes. The oral ends of the areas between the transverse and sagittal ribs are extended to form four flaps, called the "auricles." There are no tentacles nor tentacle-sheaths of the ordinary kind in the adult form; but numerous tentilla, similar in some respects to the pinnae of the tentacles of other Ctenophora, form a fringe round the margin of the auricles and the peristome. A single pair of long, filamentous, non-retractile tentacles arise from the sides of the peristomium in _Eucharis {419}multicornis_. These tentacles have no sheaths, and do not bear pinnae. They are probably not homologous with those of other Ctenophora.

The characters that separate the families of Lobata are chiefly those of varying size, shape, and position of the peristomial lobes and auricles. In the Lesueuriidae the peristomial lobes are rudimentary; in the other families they are moderately or very large. In the Bolinidae the auricles are short, but in most of the other families they are long and ribbon-like. In _Eucharis_ they can be spirally twisted in repose.

The modifications of the external form seen in the Lobata are accompanied by some modifications of the internal structure. Among these, perhaps the most interesting is a communication between the transverse longitudinal and the paragastric canals, and the long convoluted tubes given off to the peristomial lobes by the sagittal longitudinal canals. Very little is known about the life-history and development of most of the Lobata, but Chun has shown that in _Eucharis_ and _Bolina_ there is a Cydippiform larval stage which produces ripe ova and spermatozoa. This is followed by a period of sterility, but when the adult characters are developed they become again sexually mature. To this series of sexual phenomena the name "Dissogony" is given.

The order contains only fifteen genera, but they are usually arranged in the following eight families:—

1. LESUEURIIDAE. _Lesueuria._
2. BOLINIDAE. _Bolina, Bolinopsis._
3. DEIOPEIDAE. _Deiopea._
4. EURHAMPHAEIDAE. _Eurhamphaea._
5. EUCHARIDAE. _Eucharis._ {420}
6. MNEMIIDAE. _Mnemia, Mnemiopsis._
7. CALYMMIDAE. _Calymma._
8. OCYROIDAE. _Ocyroe._

Most of these Ctenophores occur in the warm and tropical seas; but _Bolina_ is found occasionally at Plymouth in the month of May, on the west coast of Ireland, and at other stations on the British coasts. _Eucharis_ is regarded as one of the most beautiful of the Phylum. A swarm, some miles in length, of large specimens of _E. multicornis_ was met by the Plankton Expedition in the south equatorial current of the Atlantic during the month of September.

ORDER III. CESTOIDEA.

In this order the body is so much compressed in the transverse plane and elongated in the sagittal plane that it assumes the shape of a long narrow band or ribbon. The tentacular sheaths are present but the tentacles are degenerate in the adult. The tentacular functions are performed by numerous tentilla situated in long grooves extending along the whole length of the oral side of the band-like body. The transverse ribs are reduced; the sagittal ribs extend along the whole of the aboral side.

FAM. CESTIDAE.—This is the only family of the order. _Cestus veneris_, the Venus's girdle of the Mediterranean Sea, is also found in the Atlantic Ocean, and specimens belonging to the same genus, but probably to a different species, occur as far north as the White Sea. Some of the larger specimens are considerably over 1 metre in length.

_C. pectenalis_ was found in abundance off one of the Maldive Islands [431] and differs from _C. veneris_ in having a large and {421}prominent orange patch at each end of the body. It is said to be extremely graceful in the water, moving with slow, ribbon-like undulations, and shining in the sunlight with a violet iridescence. _Vexillum_, from the Mediterranean Sea and Canary Islands, is rather more pointed at the extremities than _Cestus_, and differs from it in some important anatomical characters.

ORDER IV. PLATYCTENEA.

This order has been constituted for two remarkable genera, in which the oro-apical axis is so much reduced that distinct dorsal and ventral surfaces can be distinguished.

There is a single pair of long milky-white tentacles capable of complete retraction into tentacular sheaths.

FAM. 1. CTENOPLANIDAE.—_Ctenoplana_ was discovered by Korotneff in 1886 floating with the Plankton off the coast of Sumatra. In 1896 Willey [432] discovered four specimens on a cuttle-bone floating off the coast of New Guinea. To these authors we are indebted for the only accounts of this animal that have been published.

When the _Ctenoplana_ is creeping on the bottom of a dish or with its dorsal side downwards on the surface film of the water, it has the form of a flattened disc with a notch on each side. On the upper or dorsal surface eight short rows of ctenophoral plates may be seen, and in a position corresponding with the two notches in the margin of the body are situated the two sheaths from which the long pinnate tentacles protrude. In the exact centre of the dorsal surface is situated the statolith, supported by stiff processes from adjacent cells; and forming a circlet round the statolith there is a row of short ciliated tentacles. These tentacles, however, when examined carefully in the living animal, are found to be arranged in two sets of about nine in each, separated by narrow gaps on each side, the gaps corresponding in position with the axis through the tentacles.

When the animal is swimming it assumes a helmet-shape by depressing the sides of the body like a pair of flaps on the tentacular axis, and then the ctenophoral plates come into play and produce the progressive movements of the animals. The pinnate tentacles are opaque white in colour, and have peculiar serpentine {422}movements. Very little is known at present concerning many details of the internal anatomy, but there is one point of considerable theoretical interest—namely, the presence of definite male genital ducts.

Three of Dr. Willey's specimens were mottled with a green pigment, whereas his fourth specimen and Korotneff's only specimen were mottled with a red pigment. It has yet to be determined whether the differences which have been observed in the individual specimens are of specific value.

FAM. 2. COELOPLANIDAE.—_Coeloplana_ was originally discovered by Kowalevsky in the Red Sea, but has recently been found by Abbott [433] on the coast of Japan.

The Japanese species are found principally on encrusting Algae, _Zostera_, _Melobesia_, etc., which they resemble very closely in colour. The Red Sea species is, according to Kowalevsky, ciliated all over, but the Japanese species are ciliated only on the ventral surface. As in _Ctenoplana_, the body of _Coeloplana_ is a flattened disc with a notch at each end of the tentacular axis, when creeping; but _Coeloplana_ does not swim, nor at any time does it assume a helmet-shape. The tentacles are very long and of a chalky-white colour. They can be retracted into tentacle-sheaths. When the animal is excited it throws out the whole tentacle in a cloud of white filaments, "and to watch it at such a time, shooting out and retracting the tentacles, moving along the side of the aquarium like a battleship in action is truly a remarkable spectacle."[434] On the dorsal side of the body there is a series of processes which are called the dorsal tentacles. The statolith is very small, and is not surrounded by sensory processes as it is in _Ctenoplana_. There are no ctenophoral plates. The colours of the Japanese {423}species are scarlet or carmine red and dirty brown or brownish yellow. They are from 1 to 2 centimetres in diameter.

CLASS II. NUDA

Ctenophora without tentacles.

FAM. BEROIDAE.—_Beroe_, the only genus of this family and class, differs from other Ctenophora in several important particulars. There are no tentacles, and the stomodaeum is so large that the body-form assumes that of a thimble with moderately thick walls. The infundibulum is small. The paragastric and longitudinal canals give rise to numerous ramifications which form a network distributed throughout the surface of the body. The statolith is unprotected by a dome, and the polar fields are bordered by a number of small branching papillae. The eight ribs extend for nearly the whole length of the body. _Beroe_ is almost cosmopolitan, and is frequently found at the surface of the sea in great numbers. _B. ovata_ is found off the Shetlands, Hebrides, and west coast of Ireland, but is rare on the east coast of the British Islands and in the English Channel. At Valencia it is common in August and September, and sometimes reaches the great size of 90 mm. in length by 50 mm. in breadth. It is usually of a pale pink colour.