XI 



POLYZOA 



395 



similar to the Cyphonautes. It has a bivalve shell, a well-developed 

 pyriform organ, and a complete set of muscles ; but the alimentary 

 canal is somewhat degenerate, the intestine being wanting. The 

 corona forms a complete ring without cross ridges. 



In some species of Alcyonidium, such as Alcyonidium polyoum, 

 described by Harmer(1887) 

 (Fig. 316, A), further de- 

 generacy can be seen ; a 

 stomodaeum and stomach 

 alone are present, as in the 

 larva of Flustrella, but the 

 bivalve shell is gone, and 

 the apical organ is a wide, 

 flat disc. The corona con- 

 sists of a single ring of 

 large ciliated cells. 



When we pass to Cheilo- 

 stomata, like Lepralia, 

 Bugula, etc., we find that 

 the gilt has entirely 

 disappeared, and is repre- 

 sented by a mass of mesen- 

 chyme cells. The corona 

 of Lepralia resembles that 

 of Alcyonidium, but that 

 of Bugula consists of 

 enormously tall cells, each 

 extending through the 

 whole height of the larva 

 (Fig. 316, B). 



In the Cyclosfcomata 

 there is also a gutless 

 larva, but now the apical 

 organ is represented by a 

 deep invagination devoid 

 of sense cells ; the corona 

 is represented by a broad 

 belt of ciliated cells of 

 comparatively small size, 

 and the pyriform organ is 

 absent. 



Finally in the freshwater Phylactolaemata, where, as in many 

 other freshwater animals, there is an extremely shortened develop- 

 ment, we find an oval larva, most of whose surface is covered with 

 fine cilia, but which has an invagination at the anterior pole whilst 

 the posterior pole is glandular. The broad ciliated band represents 

 the corona, the apical invagination the apical organ, which is, 

 however, entirely devoid of sense-hairs, and from which one or two 



Fig. 316. — Two degenerate types of larvae of Ecto- 

 proct Polyzoa. (Combined from figures given by 

 Korschelt and Heider. ) 



A, optical section of tlie larva of Alcyonidium polyoum. 

 B, optical section of the larva of Bugula plumosa. The dark 

 streaks represent coronal cells lying between the reader 

 and the median plane of the larva, ap, apical disc ; ap.g, 

 ganglion cells beneath the apical disc ; co, corona ; i.s, 

 internal sac ; m, month ; p.Oy pyriform organ ; st, stomach. 



