20 



BRYOZOA ECTOPROCTA. 



an ectodermal invagination, into a wide gastric cavity that ends 

 blindly. A comparison with the larvae of Tendra and of Cypho- 

 nautes renders it probable that the part lying behind the aperture of 

 the sucker corresponds to the anal region. This distinctly developed 

 alimentary canal appears to be a vestigial organ, there being no 



indication that it 

 actually functions. 

 It degenerates in 

 the further course 

 of larval life. 



The thickness 

 of the epithelium 

 which lines the 

 mantle-cavity (p) 

 is remarkable, and 

 contrasts with the 

 somewhat thin 

 ectodermal layer 

 of the retractile 

 disc. Closely con- 

 nected with the 

 latter layer, in the 

 anterior part of 

 the larva, there is 

 a cell-mass (Fig. 

 7 B, g) which 

 Harmer (No 13) 

 regards as the 

 brain. This author 

 observed, arising 

 within this cell-mass, fibrous strands, some of which are said to 

 become connected with cells of the ectodermal furrow, while others 

 can be followed into the neighbourhood of the large coronal cells. 

 Should these statements be confirmed, we should appear justified in 

 considering the pyriform organ as a sensory organ, whereas until now 

 its histological constitution seemed to point to the conclusion that it 

 functioned as a gland. The presence of a larval brain comparable in 

 position to the neural plate would be of importance in tracing the 

 l-Vtoproctous larvae back to the Trochophore. The statements of 

 Hakmku hare recently been confirmed by the researches of Prouho- 

 (Na. 28) in connection with Flustretta, but on account of the 



Sfer-C 



b5\P 



771 



Fig. 7. — A, larva of Alcyonidium mytili (after J. Barrois). B, 

 longitudinal vertical section through the larva of Alcyonidium 

 (diagram constructed after the figures of Harmer). c, coronal 

 cells ; g, brain (?) ; m, oral aperture ; o, pyriform organ ; p, 

 mantle-cavity ; r, retractile disc ; s, sucker. 



