130 HIUOSIII OlISIIIMA 



1, 2, 20, 22, 23). The cause is unknown to us ; still, I think 

 there is hardly any doubt as to its being due to artificial 

 conditions. Too large a number of diatoms or bacteria in the 

 vessel in which the larvae have been kept may cause this. 

 Shortly afterwards the pore and canal can regenerate (Cases B, 

 23) and the revived development of the whole water-vascular 

 system follows. In other instances no second pore was formed, 

 and degeneration of the system soon set in (Cases 1, 8, 10, 11). 

 The presence of the right pore-canal side by side with the 

 left is a constant and normal character in the larva of Mel lit a 

 pen tap or a (Grave, 9, p. 42 ; and also his former paper, 

 1902, p. 58). The same is not common in Echinus 

 ni i 1 i a. r i s ; still, it has been recorded by M a c B r i d e in 

 a larva which was otherwise quite normal (15, p. 339). 

 Although the presence of two pore-canals is a very common 

 occurrence among double-hydrocoele larvae (Cases 4, 6, 16, 

 17, 19, 20) it seems by no means to be a necessarily associated 

 feature. In starfish the occurrence of the double dorsal pore 

 has never been seen even among double-hydrocoele larvae 

 (Gemmill, 5, p. 230; 7, p. 31 ; 8, p. 62). To such an 

 important difference found between these two classes let us 

 return later (p. 142). According to Eunnstrom the forma- 

 tion of the dorsal pore and pore-canal seems to be a self- 

 differentiation (25, p. 301). 



(d) S t o n e - c a n a 1. — This is the part which at first connected 

 the hydrocoele bud with the main body of the anterior coelom. 

 This canal is sometimes found doubled, being caused from 

 either its defective origin (Case 7) or abnormal regeneration 

 (Case B). When degeneration takes place, probably due to 

 the lack of communication wdth the exterior, it begins from 

 that end which is adjacent to the axial sinus (Cases 1, 8, 11). 



(e) Hydrocoele. — It is a well-known fact that the right 

 coelomic sac has in normal larvae the potentiality of producing 

 a sac which is homologous with the left hydrocoele. Such 

 a special organ-forming substance seems to be located especially 

 at the place where the coelomic sac has to divide later into the 

 anterior and posterior coeloms. We see from MacBride's 



