172 



Mr. H. Ohshima. Reversal of 



though further development of both these structures are mutually dependent 

 ((10), p. 305 ; (12), pp. 9-11, 13-14). 



(e) Peculiar differentiation of the right posterior coelom, to share with the 

 hydrocoele and amniotic invagination in the formation of an echinus-rudiment, 

 exactly as its left side fellow normally does. 



(/) Formation of pedieellarise on the left side. This occurs in the reversed 

 larvae (two cases in Strongylocentrohis lividus by Eunnstrom (9), p. 2,(9), p. 10, 

 (12), p. 422) and in some double-hydrocoele ones {Echinus miliaris MacBride 

 (6), p. 343). They probably owe their existence to the echinus-rudiment 

 developed on the opposite side. In the complete absence of hydrocoele from 

 both sides pedicellarise cannot usually be formed at all (exceptional cases are 

 recorded by Eunnstrom (11), pp. 269-70, text-figs. 33-35). 



From these data above enumerated, if adequately combined, the following 

 changes seem to be possible : — 



Let us start with a young normal larva, in which hydrocoele, axial sinus, 

 pore-canal, and dorsal pore are all formed on the left side. An amniotic 

 invagination may already be formed on the left side, and the right anterior 

 coelom may be provided with a pore-canal. 



Now, the dorsal pore of .the- left side happens to become obliterated, and 

 is hence followed by the arrest of development of the hydrocoele and its 

 subsequent degeneration. Two courses are here open : — 



A. The right anterior coelom begins to exhibit its unusual activity in 

 producing a right hydrocoele, which acquires through a pore-canal a communi- 

 cation with the exterior. 



B. The right anterior coelom does not become active (presumably from 

 want of sufficient nutrition). The result is the complete absence of hydro- 

 coele from both sides. 



The further fate of larvae in which the course of events has been that 

 indicated by A wiU be one of the following three : — 



1. Appearance of a new dorsal pore on the left side, so that the left hydro- 

 coele regains the power to develop further. If well fed, the hydrocoele on 

 each side will continue to develop side by side, so as to give rise to a double- 

 hydrocoele larva. 



2. Fusion of the anterior coeloms of both sides takes place, so as to enable 

 the left hydrocoele to communicate with the exterior and to continue further 

 development. The result is also a double hydrocoele. 



3. No reappearance of a second dorsal pore on the left side nor fusion of 

 the anterior coeloms takes place. The left water- vascular system will conse- 

 quently degenerate quickly, while the right one will develop like the normal 

 left. A larva with the situs inversus is the result. 



