176 CTENOPHORA. 



consist, as in the Acraspcdote Medusa: and Actinozoa, of two sections: (i) 

 A true hypoblastic section consisting of the infundibulum and the gastro- 

 vascular canals derived from it ; and (2) an epiblastic section the stomo- 

 d.i-um forming the stomach. 



The observations of Kowalevsky on the alimentary system do not wholly 

 tally with those of Agassiz. He finds that the oral side of the embryo 

 becomes hollowed out, and that the hollow, lined by flattened cells, becomes 

 constricted off as the infundibulum, from which the radial canals subse- 

 quently grow out. To the infundibulum there leads a narrow canal lined by 

 a columnar epithelium which becomes the gastric cavity. 



While the alimentary canal is becoming formed a series of 

 important changes takes place in other parts of the embryo. 

 The rows of locomotive paddles first appear as four longitudinal 

 equidistant linear thickenings of the epiblast near the aboral pole 

 (fig. 83 D). On the projecting surface of these ridges stiff cilia 

 appear which coalesce together to form the paddles. While the 

 embryo is still within the egg the rows of paddles are quite short 

 and also double. There arc in Pleurobrachia about eight or nine 

 pairs of paddles in each row. Each double row eventually sepa- 

 rates into two. 



In all the forms except the Eurostomata (Beroe) two tentacles 

 grow out as thickenings of the epiblast (fig. 84 B, /.). They are 

 placed at the opposite poles of the long transverse axis of the 

 embryo. 



A process of the contractile gelatinous tissue of the body, the 

 origin of which is described below, makes its way, according to 

 Kowalevsky, into the tentacles. 



The central apparatus of the nervous system and the otoliths 

 are formed at the aboral pole from a thickening of the epiblast, 

 but the full details of their formation have not been elucidated. 

 It may be well to preface my account of their development with 

 a short statement of their adult structure. 



They consist in the adult of a vesicle with a ciliated lining situated at 

 the bifurcation of the two anal tubes, and of certain structures connected 

 with this vesicle. From the floor of the vesicle is suspended a mass of 

 otoliths by four leaf-like bodies known as suspenders. The roof is very 

 tc ami IMS the form of a four-sided pyramid. Six openings lead into 

 the vesicle. Through four of these, placed at the four corners, there pass 

 out four ciliated grooves continuous with the suspenders. These grooves, 

 after leaving the otolithic vesicle, bifurcate and pass to the eight rows of 

 paddies. At the two sides the walls of the vesicle are continuous with two 



