5 I O POLYZOA 



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 " Trocho- 

 sphere " larva can be taken to represent it in a much simplified 

 condition. Such a view harmonises well with the great antiquity 

 of the 1'olyzoa. 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 consider- 

 able significance that this type of larva is known to occur only 

 in those species of Membranipora (Electro), Alcyonidium, and 

 Hypopliorella, which lay eggs. 1 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 (Electro) pilosa. Whilst this larva is provided 

 with a well-developed alimentary canal, those of most other Ecto- 

 procta 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 Pedicdlina 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 Cyj)honautcs, itself allied to the larva of the 



1 Prouho, loc. ciL 



