146 



CTENOPHORA. 



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 cei'taiu 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 delicate and has 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 paddles. At the two sides the walls of the 

 vesicle are continuous with two thickened ciliated plates with swollen 

 edges, opposite the centres of which are two lateral openings into the 

 vesicle, completing the six openings. Through the lateral openings 

 the sea water is driven by the action of the cilia of the plates. 



The development of these parts is as follows In the aboral 

 thickening of epiblast a cavity makes its appearance, the walls 

 of which constitute the rudiment of the otolithic vesicle (fig. 83 B 

 and C, s.c.). The roof of the cavity is extremely delicate. On each 

 side of it a thickening of cells becomes established, regarded by 

 Kowalevsky as the rudiment of the nervous ganglia. These thicken- 

 ings appear to give origin to the lateral ciliated plates. The otoliths 

 arise from cells at four separate points at the corners of the ciliated 

 plates opposite the rows of paddles (fig. 84 A, ot.}. 



In Pleurobrachia there is at first only one otolith at each corner. 

 The otoliths are gradually transported towards the centre of the vesicle 

 (fig. 84i B, ot.} and are there attached, though the four leaf-like 



suspenders do not arise till very late. 

 The otoliths go on increasing in num- 

 ber throughout life. 



The gelatinous tissue of the Cteno- 

 phora appears as a homogeneous layer 

 between the epiblast and the yolk- 

 cells, and is probably homologous with 

 the layer formed in the same situation 

 in all other ccelenterate forms. Into 

 the layer a number of anastomosing 

 cells, mainly derived from the epiblast, 

 though according to Chun (No. 174) 

 also in part from the hypoblast, make 

 their way. These cells would appear to be mainly, if not entirely 



A 



FIG. 84. Two STAGES IN THE DE- 

 VELOPMENT OF PLEUROBEACHIA RHOD- 

 ODACTYLA. (After Agassiz.) 



ot. otolith; t. tentacle. 



