140 HIROSHI OHSHIMA 



degeneration of the left water-vascular system. Two courses 

 are here open : A. The right anterior coelom begins its unusual 

 development to produce a right hydroco(fle, which acquires 

 communication with the exterior through a pore-canal. 

 B. The right anterior coelom does not become active either 

 from very weak disposition of the right anterior coelom or, 

 more probably, from want of sufficient nutrition. The result 

 is the total absence of hydrocoele from both sides. 



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

 been that indicated by A will be one of the following three : 



1. Appearance of a new dorsal pore on the left side which 

 revives the power of the left hydrocoele 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. Axial sinuses of both sides come in contact with each other 

 and then unite, thus making the left hydrocoele regain its 

 communication with the exterior and enabling it to develop 

 further. The result is also a double hj^drocoele. 



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

 axial sinuses takes place. The left water-vascular system will 

 then degenerate quickly, while the right one will develop like 

 the normal left. A larva with situs inversus is the 

 result. 



In both the courses of events indicated by 1 and 2 the 

 following three conditions may possibly arise, according to the 

 different stages at which the right hydrocoele had arrived, 

 when the recovery of the left hydrocoele took place : 



(a) The recovery of the left hydrocoele takes place before the 

 right hydrocoele attains a size equal to the left. The period 

 during which the hydrocoele is deprived of communication 

 with the exterior is very short. Under such a condition the 

 result is a larva whose left hydrocoele or echinus-rudiment is 

 larger or more advanced than that of the right side. This is 

 very frequently met with among double-lwdrocoele larvae. 



(h) The left hydrocoele recovers at the time when the right 

 one attained a size about equal to it. The larva developed 

 under such a condition has two hydrocoeles or echinus-rudiments 



