Chapter XXI: Echinodermata (continued): Development and Phylogeny 601 (6)

In _Carchesium_ (Fig. 51) the muscular threads of each cell are separate, while in _Zoothamnium_ they are continuous throughout the colony. _Epistylis_ has a solid, rigid stalk, and may give rise to branching colonies, which often infest the body of the Water-Fleas (Copepoda) of the genus _Cyclops_. _Opercularia_ is characterised by the depth of the gutter, the height of the collar, and the tapering downward of the elongated disc. _Vaginicola_, _Pyxicola_, _Cothurnia_, _Scyphidia_, all inhabit tubes, some of extreme elegance. _Ophrydium_ is a colonial form, found in ponds and ditches, resembling _Opercularia_, but inhabiting tubes of jelly[176] that coalesce by their outer walls into a large floating sphere; it usually contains the green symbiotic Flagellate _Zoochlorella_. _Trichodina_ is free, short, and cylindrical, with both wreaths permanently exposed, and is provided with a circlet of hooks within the aboral wreath. It is often parasitic, or perhaps rather epizoic, on the surface of _Hydra_ (see p. 254), gliding over its body[177] with a graceful waltzing movement; it occurs also in the bladder and genito-urinary passages of Newts, and even in their body-cavity and kidneys.

II. SUCTORIA = TENTACULIFERA

_Infusoria with cilia only in the young state,[178] without mouth or anus, but absorbing food (usually living Ciliates) by one or more tentacles, perforated at the apex; mostly attached, frequently epizoic, rarely parasitic in the interior of other Protozoa._

{159}_Acineta_, Ehrb. (Fig. 61, 2); _Amoebophrya_, Koppen; _Choanophrya_,
Hartog (Fig. 62); _Dendrocometes_, St. (Fig. 61, 4); _Dendrosoma_, Ehrb.
(Fig. 61, 9); _Endosphaera_, Engelm.; _Ephelota_, Str. Wright (Fig. 61,
5, 8); _Hypocoma_, Gruber; _Ophryodendron_, Cl. and L. (Fig. 61, 7);
_Podophrya_, Ehrb. (Fig. 61, 1); _Rhyncheta_, Zenker (Fig. 61, 3);
_Sphaerophrya_, Cl. and L. (Fig. 61, 6), _Suctorella_, Frenzel;
_Tokophrya_, Bütschli.

This group, despite a superficial resemblance to the Heliozoa, show a close affinity to the Ciliata; the nuclear apparatus is usually double though a micronucleus is not always seen; the young are always ciliated, and the mode of conjugation is identical in all cases hitherto studied. Most of the genera are attached by a chitinous stalk (Fig. 61), continued in _Acineta_ into a cup or "theca" surrounding the cell. The pellicle is firm, often minutely shagreened or "milled" in optical section by fine radial processes, whether superficial rods or the expression of the meeting edges of radial alveoli is as yet uncertain. The pellicle closely invests the ectosarc, is continued down into a tubular sheath, from the base of which the tentacle rises, and upwards to invest the tentacle, and is even prolonged into its cavity in _Choanophrya_, the only genus where the tentacles are large enough for satisfactory demonstration. These organs may be one or more, and vary greatly in character. They may be (1) pointed for prehension, puncture, and suction (_Ephelota_, Fig. 61, 5); (2) nearly cylindrical, with a slightly "flared" truncate apex (_Podophrya_, Fig. 61, 1_a_); (3) filiform with a terminal knob; (4) "capitate" (_Acineta_, Fig. 61, 2); (5) bluntly truncate and capable of opening into a wide funnel for the suction of food[179] (_Choanophrya_, Fig. 62; _Rhyncheta_, Fig. 61, 3). Their movements, too, are varied, including retraction and protrusion, and a degree of flexion which reaches a maximum in _Rhyncheta_ (Fig. 61, 3), whose tentacle is as freely motile as an elephant's trunk might be supposed to be were it as slender in proportion to its length. They are continued into the body, and in _Choanophrya_ may extend right across it. In _Podophrya_ trold the pellicle rises into a conical tube about the base of the tentacle, which is retracted through it completely with the prey in deglutition. In _Dendrocometes_, _Dendrosoma_, and _Ophryodendron_ (Fig. 61, 4, 9, 7), the tentacles arise from outgrowths of the cell-body.

{160}

{161}The mechanism of suction is doubtful; but from the way particles from a little distance flow into the open funnels of _Choanophrya_, it may be the result of an increase of osmotic pressure. The external pellicle of the tentacles is marked by a spiral constriction,[180] which may be prolonged over the part included in the sheath. The endosarc is rich in oil-drops, often coloured, and in proteid granules which sometimes absorb stains so readily as to have been named "tinctin bodies." It usually contains at least one contractile vacuole.

In _Dendrocometes_ (and perhaps others) the whole cell may become ciliated, detach itself and swim off; this it does when its host (_Gammarus_) moults its cuticle.

In fission or budding we have to distinguish many modes. (1) In the simplest, after the nuclear apparatus has divided, the cell divides transversely; the distal half acquires cilia and swims off to attach itself elsewhere, while the proximal remains attached. The tentacles have previously disappeared and have to be formed afresh in both. (2) More commonly fission passes into budding on the distal face; a sort of groove deepens around a central prominence which becomes the ciliated larva (Fig. 62, _em_); the tentacles of the "parent" are retained. This process passes into (3) "internal budding," where a minute pit leads into a bottle-shaped cavity.[181] (4) Again, the budding may be multiple, the meganucleus protruding a branch for each bud, while the micronucleus, by successive divisions, affords the supply requisite. _Sphaerophrya_ (Fig. 61, 6) and _Endosphaera_ multiply freely by fission within their Ciliate hosts, and were indeed described by Stein as stages in their life-cycle. Conjugation of the same type as in most Ciliates has been fully worked out in _Dendrocometes_ alone, by Hickson,[182] who has found the meganuclei (though destined to disorganisation) conjugate for a short time by the bridge of communication before the reciprocal conjugation of the micronuclei.

We have referred to the endoparasitism of two genera. _Amoebophrya_ lives in several Acanthometrids, and in the aberrant Radiolarian _Sticholonche_ (see p. 86). The attached species are {162}some of them indifferent to their base; others are only found on Algae, or again only epizoic on special Metazoan hosts, or even on special parts of these. Thus _Rhyncheta_ is only found on the couplers of the thoracic limbs of _Cyclops_, and _Choanophrya_ on the ventral surface of its head and the adjoining appendages.

We owe our knowledge of this group to the classical works of Ehrenberg, Claparède and Lachmann, Stein, R. Hertwig, and Bütschli. Plate has shed much light on _Dendrocometes_, and Hickson has studied its conjugation. Ischikawa[183] has utilised modern histological methods for the cytological study of _Ephelota bütschliana_. René Sand has written a useful, but unequal, and not always trustworthy monograph of the Order,[184] containing an elaborate bibliography.

PORIFERA (SPONGES)

BY

IGERNA B. J. SOLLAS, B.SC. (LOND.)

Lecturer on Zoology at Newnham College, Cambridge.

{165}CHAPTER VII

PORIFERA (SPONGES)[185]