The larvae of the Entoprocta (Fig. 252, A) resemble the so-called "Trochosphere" of Polychaeta (see p. [274]). The common characters shared by the larvae of Chaetopoda, Echiuroid Gephyrea, Mollusca, and Polyzoa, and by adult Rotifera, may well point to the derivation of these groups from a common ancestor. On this assumption, it is possible that the Polyzoa have been derived from forms which existed long ages ago, which combined the common characters of these groups, and the structure of which we can picture to ourselves only so far as the "Trochosphere" larva can be taken to represent it in a much simplified condition. Such a view harmonises well with the great antiquity of the Polyzoa. Certain Ectoproct forms have a larva, known as Cyphonautes (Fig. 252, B), which closely resembles the larval form of the Entoprocta; and it is a fact which probably has considerable significance that this type of larva is known to occur only in those species of Membranipora (Electra), Alcyonidium, and Hypophorella, which lay eggs.[[577]] This may perhaps be regarded as a primitive form of development which has been lost in species in which development takes place inside the parent. Cyphonautes compressus (Fig. 252, B), one of the commonest objects taken in the surface-net off our own coasts, is the larva of Membranipora (Electra) pilosa. Whilst this larva is provided with a well-developed alimentary canal, those of most other Ectoprocta possess a mere rudiment of this structure, and depend for their nutrition either on yolk present in the egg or on material supplied by the parent. In most cases the mature larva has no recognisable trace of a digestive system; and, although it has a free-swimming period, it does not become truly pelagic.
The alimentary canal of the larva of Pedicellina is known to persist in the primary individual of the colony. In all other known cases, even in that of Cyphonautes, the larva at fixation loses practically all its internal organs, and becomes a mere body-wall containing a mass of degenerated larval tissues. It is in fact a zooecium containing a "brown body." A polypide-bud is now developed, the body-cavity appears as the result of the shrinkage of the "brown body," and the primary individual of the colony is thereby established.
The larvae of the Ectoprocta form a tolerably complete series, starting from Cyphonautes, itself allied to the larva of the Entoprocta, and ending with the Phylactolaemata. Alcyonidium (Fig. 253, B) possesses a rudimentary alimentary canal,[[578]] although the most conspicuous structures are those connected with the fixation and other phenomena of larval life. The larvae of many of the encrusting Cheilostomes (Fig. 253, A) resemble that of Alcyonidium, while those of Bugula, Scrupocellaria, etc., belong to a type easily derivable from that of the encrusting forms. The branching Ctenostomes (Bowerbankia, etc.) have a larva which may be regarded as derived, along slightly different lines, from that of Alcyonidium. The Cyclostomata and the Phylactolaemata have the most modified forms of larva. That of the former group may owe some of its peculiarities to the occurrence of a remarkable process of embryonic fission, which takes place in the ovicell, and as the result of which each egg gives rise to a large number of larvae.[[579]] The Phylactolaemata have a larva which is not unlike that of Bowerbankia.
Fig. 253.—A, Aboral view of free larva of Lepralia foliacea Ell. and Sol.; a, long cilia of pyriform organ; g, aboral groove: B, longitudinal section of embryo of Alcyonidium, × 135; c, ciliated ring; g, aboral groove; m, mouth; n, nervous system; p, "pyriform organ," of unknown function; s, "internal sac" or "sucker," by which fixation is effected; st, stomach.
We have seen that the larva at fixation becomes a zooecium, which in the Gymnolaemata forms a polypide-bud after fixation. The peculiarities of the Phylactolaematous larva may be explained by assuming that it becomes a zooecium while it is still free-swimming. Thus the larva of Plumatella develops one or sometimes two polypides, which actually reach maturity before fixation takes place. That of Cristatella develops from two to twenty[[580]] polypides or polypide-buds at the corresponding period, and it is in fact a young colony while still free-swimming.
Now in most colonial animals, such as Coelenterates and Ascidians, the larva metamorphoses itself into a temporarily solitary animal, which then gives rise to the remainder of the colony by budding. The majority of the Gymnolaemata behave in this way; while the Phylactolaemata may not only develop a multiplicity of polypides in their larval stage, but the individuality of the zooecia is then just as much obscured as in the adult state. These facts are more easily explained if we assume that Cristatella is the end-point in a series than if we suppose it to be a starting-point.
On the view maintained by many authorities, that the Polyzoa are related, through Phoronis, with the Gephyrea and the Brachiopoda, we should expect to find in those Polyzoa which most closely resemble Phoronis in their adult state—that is to say in the Phylactolaemata—some indications of affinity to that animal in their development. This is emphatically not the case. The hypothesis that the Phylactolaemata are related to Phoronis leads, moreover, to the improbable conclusion that the similarities between the Entoproct-larva and Cyphonautes, on the one hand, and the Trochosphere larva of Polychaeta, on the other hand, is entirely superficial and meaningless. In spite, therefore, of the similarity between Phoronis and a single individual of the Phylactolaemata, and in spite of the marked resemblance between its nephridia and structures which have been described in Cristatella[[581]] and Pectinatella[[582]] the comparative study of the development appears to indicate that the resemblances between Phoronis and the Phylactolaemata are the result of a coincidence rather than of any close relationship.
A few points connected with the metamorphosis of the Polyzoa deserve more special notice. There is generally great difficulty in persuading larvae to fix themselves when kept in a small quantity of water, which becomes over-heated in the air of a laboratory. The difficulty may be surmounted by placing colonies containing embryos, together with some clean pieces of the seaweed on which the adults are habitually found, in a vessel closed by a piece of fine muslin, and by leaving the vessel attached to a buoy or in a deep tide-pool. The larvae being without an alimentary canal, fix themselves, after a very short free life, on the seaweed.
It is probable that a great struggle for existence normally takes place at the commencement of the metamorphosis. Any one who will examine, in June or July, rocks covered by Fucus on which Flustrella hispida is growing, will probably find numerous young fronds of Fucus, from half an inch to an inch or two in length, growing under the shelter of the older fronds. The bivalve larvae of Flustrella show a marked preference for fixing on these young fronds—perhaps in order that the duration of life of the colony may coincide with that of the Fucus—and these young fronds are commonly covered by very numerous recently-fixed larvae, and by young colonies of various ages. Or, it is easy to observe, by placing pregnant colonies of Bowerbankia in a vessel of water, that the larvae, which are hatched out in thousands, fix themselves in dense masses on certain parts of the wall of the vessel. It is clear that but a small proportion of these larvae will find room for further development.