TYPE WHICH DEVELOPS AN ALIMENTARY CANAL. 19 



Avill be followed by the account of its transformation into the 

 attached form, as far as this latter is accurately known. 



1. Type whicli develops an Alimentary Canal. 



We shall commence our study of the Ectoproctous larvae with 

 the consideration of a few isolated forms, which, in external struc- 

 ture, do not greatly differ from the typical larva (especially from 

 that of the Escharina), but which in the higher differentiation of 

 their internal organs show a more primitive condition. In the 

 possession of a well-developed (though also to some extent function- 

 less) alimentary canal especially, these larval forms afford important 

 points of comparison with the larvae of other groups, whereas, in 

 most other Ectoproctous larvae, the primary entoderm gives rise 

 merely to a parenchyma (central tissue), which in many forms 

 contains a yolk-mass. 



a. Larva of Alcyonidium. 



The larva of Alcyonidium (Fig. 7) has already been described and 

 figured by Farre and others, and later by Barrois (^o. 6), its 

 internal structure having been sj^ecially investigated by Harmer (jSTo. 

 13). This larva differs little externally from the type which we shall 

 find retained in the Escharina. When viewed from above, it appears 

 perfectly spherical ; when seen from the side (Fig. 1 A) the oral 

 surface is distinguished by its greater convexity from the flat 

 aboral side. Almost the whole of the latter is occupied hy the 

 retractile disc (Fig. 7, r), Ijordered Avith stift* cilia, this organ being 

 •separated by a circular depression (inantle-cavity, jy) from the ciliated 

 /■orona (c) which is composed of large cells. On the oral surface, in 

 the anterior region of the larva, Ave can recognise the a7iterior 

 ectodermal furrow (o), at Avhose edge are cilia Avhich, anteriorly, 

 become grouped together to form a ciliated tuft (phomet). In the 

 posterior part of the larva the sucker-invagination (.s) is visible. 

 Tavo pairs of flagella belonging to the posterior region of the oral 

 .surface should also be mentioned. 



In a longitudinal section (Fig. 7 B), Ave recognise first the large 

 cells of the ciliated ring (c), then, in the anterior part of the oral 

 ■surface, the pyriform. organ (o) lying at the l^ase of the ectodermal 

 groove, consisting of elongated ectodermal cells that possess a 

 peculiar histological character. BetAveen this organ and the aperture 

 of the sucker (s), Avhich lies further back, is the oral aperture {m) 

 leading through a narroAV oesophagus, Avhich may have arisen as 



