XI 



TOLYZOA 



395 



CO 



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(188'7) 



(Fig. 316, A), further de- ap a P9 * 



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- 

 stoniata, like Lepralia, 

 Bugula, etc., we find that 

 the gut has entirely 

 disappeared, and is repre- 

 sented by a mass of mesen- 

 chyine 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 Cyclostomata 

 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 ancl the m( ''li ;in plane of the larva. ;<, apical disc ; fii>.,i, 



v . _i : 1 1 _. i j_i_ j_i_ . _ _ i .1 



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 \vith 

 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 whieh 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 the larva of Al<-i/<>i,i<Hii in //n/i/imm. 

 B, optical section of the larva of Iljujiiln jilmiuisu. The dark 

 streaks represent coronal cells lying between tin- leader 



;anglion cells beneath the apical disc; co, corona; i.a, 

 internal sac ; m, mouth ; j>.n, pyriform organ ; xt, stomach. 



