22 



BRYOZOA ECTOPROCTA. 



sharply bent round towards the middle line of the body. The shell- 

 valves meet at this point like pincers. Near the lower aperture (the 

 entrance to the atrium) the valves gape. At the aboral pole, a 

 prominence (?*) covered with stiff cilia projects from the shell, but 

 can be withdrawn into it. In this we recognise the homologue of 

 the " retractile disc." 



Some uncertainty still prevails as to the ciliated ring that runs 

 round the edge of the bell. According to Ehrenberg and Hatschek, 



there is a single com- 

 plicated ring, forming 

 many coils, but 

 Claparede and 

 Schneider, whose 

 view has been adopt- 

 ed by Prouho, think 

 they can distinguish 

 two distinct rings of 

 cilia, an anterior ring 

 in the neighbourhood 

 of the pyriform organ 

 (o) and a posterior, 

 circum-anal ring, part 

 of which sinks into 

 the atrium. 



On the oral invagi- 

 nated surface we 

 notice first, in the 

 anterior angle, a cell- 

 mass provided with 

 a tuft of cilia; this is the pyriform organ (o). According to 

 Repiachoff, two parts may be distinguished in it; an invaginated, 

 sucker-like epithelial thickening surrounded by a ring of cilia (ecto- 

 dermal furrow), and a cell-accumulation lying above this (the actual 

 pyriform organ). As in Alcyonidium, this organ is connected with 

 the retractile disc (r) by fibrous strands (/), some of which have 

 been claimed as muscle-fibres and some as nerve-fibres (Prouho). 



Cyphonautes has a fully-developed and functional alimentary canal. 

 The oral aperture (in) is found in the base of the atrium, and leads 



* [The organs s and s' are not isolated structures as represented in the figure, 

 but are situated underneath the body-wall, which latter should be indicated as 

 a line extending forwards from the anus below s and *' and joining the ciliated 

 ring near the mouth. — Ed.] 



Fig. 8.— Larva of Membranipora = Cyphonautes (combined from 

 the figures of Claparede, Schneider, and Hatschek). 

 a, atrium ; an, anal aperture ; /, fibres connecting the pyri- 

 form organ (o) and the retractile disc (r) ; m, mouth ; 

 s, glandular thickening (homologue of the sucker); s', 

 similar, smaller organ (shell-adductor muscle).* 



