SITUS INVERSUS IN ECHINOIDS 141 



equal in size. Such a case is less frequently met with than the 

 former. 



(c) The left hydrocoele recovers late when the right one is 

 in a more advanced state than it. The period during which 

 the hydrocoele is deprived of communication with the exterior 

 is here very long. The result is that the larva has the left 

 hydrocoele or echinus-rudiment smaller than the right. 

 Usually the hydrocoele and its associated structures cannot 

 remain unchanged for so long a time after being deprived of 

 its external communication. This case is therefore met with 

 very rarely. 



The above may not be the only ways of reaching the respec- 

 tive results, but probably are the commonest. Many modifica- 

 tions are naturally conceivable : for instance, the right dorsal 

 pore may be obliterated in its turn, which causes the degenera- 

 tion of the whole water-vascular system of the right side and 

 thus a normal larva will result secondarily (see Case 8). 



Let us now compare this interpretation of the occurrence of 

 the inverse situs in Echinus larvae with Spemann's 

 case of Triton larvae (29, p. 407). Though equally caused 

 by a ' defective ' development of a single organ— alimentary 

 canal in Triton and hydrocoele in Echinus— further 

 results in which the other organs become affected are different 

 in these two cases. Instead of d i s p 1 a c e m e n t of other adjoin- 

 ing organs, the arrest in development of the left hydrocoele 

 causes a new hydrocoele to appear on the other side and also 

 a new set of associated structures as a consequence. The 

 normal left hydrocoele can, if it regains its opportunity of 

 further development, produce another echinus-rudiment, so 

 as to give rise to a douljle-hydrocoele larva. Any parallel of 

 such a feature is very improbable in Triton larvae. 



There is no reason to expect that the above is equally 

 apphcable to the formation of double hydrocoele of other 

 classes of Echinoderms. Conditions may be totally different. 

 Let us, for instance, take the case of the double-hydrocoele 

 larvae of starfishes. Normally in most species of starfishes 

 the paired coelomic vesicles grow forwards, and their anterior 



J^3 



