142 IIIROSHI OHSIIIMA 



ends meet and unite in front of the larval mouth. The presence 

 of two dorsal pores is very common, but the right one gradually 

 atrophies (Gem mi 11, 5, p. 281), and still the right coelomic 

 vesicle retains its communication with the exterior through 

 the. left dorsal pore. The hydrocoele becomes later differen- 

 tiated from the middle portion of the spacious left coelomic 

 sac. In the case of the double hydrocoele the right one is 

 likewise formed from the middle portion of the right coelomic 

 sac. Among the double-hydrocoele larvae of Porania 

 pul villus and Asterias rubens Gemmill found no 

 case of the presence of double dorsal pores, in all instances 

 the left pore only being present (5, p. 230 ; 7, p. 43 ; 8, pp. 62, 

 69). Thus it is evident that the obliteration of a dorsal pore 

 has hardly any influence on the further development of the 

 hydrocoele on the same side. Under such a different con- 

 dition I suppose that the occurrence among starfish larvae 

 of the situs inversus as we find in Echinoid larvae will be 

 extremely unusual. Gemmill tried to explain the cause 

 of the double hydrocoele chiefly by the supposition that, owing 

 to the over-fed condition of the larva, its stomach becomes 

 expanded and globular, so that the ventral horn of the left 

 posterior coelom tends to fail to unite with the right middle 

 coelomic region. The latter region, being thus left isolated 

 from the posterior coeloms, produces a right hydrocoele 

 (5, p. 244 ; 8, pp. 54-5). This interpretation in its turn cannot 

 hold true in the case of those double-hydrocoele Echinoid and 

 Ophiuroid larvae, in which no such extension of the left 

 posterior coelom takes place normally (Mac Bride, 15, 

 p. 326). The discovery by MacBride (10, pp. 368-70) of 

 a double-hydrocoele larva in Asterina gibbosa, in 

 which species the egg is heavily laden with yolk, is a serious 

 objection to the hypothesis of excessive food. One feature is, 

 however, certainly common in the double-hydrocoele larvae 

 of the three different classes : namely, the temporary arrest 

 in the development of the left hydrocoele in some way or other 

 in an early stage. And this occurs more frequently under 

 artificial conditions than in nature. 



