CGELENTERATA. 



177 



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 

 thickenings 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. 84 B. ot.} and are there attached, though 

 the four leaf-like suspenders do 

 not arise till very late. The oto- 

 liths go on increasing in number 

 throughout life. 



The gelatinous tissue of the 

 Ctenophora appears as a homoge- 

 neous 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 (Chun), 

 of a contractile nature. It is probable that the great mass of the 

 gelatinous tissue of the adult is an intercellular substance derived 

 from these cells. 



The whole of the above changes are completed while the 

 embryo is still enclosed in the egg capsule. During their 

 accomplishment the oro-anal axis, which was originally very 

 short, increases greatly in length (fig. 83), so that the embryo 

 acquires an oval form similar to that of the adult. 



B. II. 12 



FlG. 84. TWO STAGES IN THE 

 DEVELOPMENT OF PLEUROBRACHIA 



RHODODACTYI.A. (After Agassiz.) 

 of. otolith ; t. tentacle. 



