SITUS INVERSUS IN BGHINOIDS 127 



stone-canal. The author suggests that this hydrocoele may 

 probably have differentiated from the posterior coelom of the 

 left side. Posterior coelom was absent at first, but appeared 

 later. Amniotic invagination formed. 



Right : anterior coelom posteriorly situated, being very 

 elongated and developed much more strongly than normal. 

 Despite the absence of hydrocoele on this side a small and 

 shallow amniotic invagination appeared later. 



[Case 23.] Strongylocentrotus livid us. Runn- 

 strom, 1918 (26), pp. 415-17, Taf. xiii, figs, la, h. Reared 

 at Monaco. 



Left : anterior coelom undifTerentiated and devoid of external 

 communication. Pore-canal ' post-generated ' and dorsal pore 

 opened on the mid- dorsal line. Hydrocoele at first remained 

 undifferentiated, but later, when amniotic invagination 

 appeared, it began to develop again. 



Right : hydrocoele not formed. Amniotic invagination 

 appeared later, but soon degenerated. 



(b) No amniotic invagination formed. 



[Case 24.] Strongylocentrotus livid us. Runn- 

 strom, 1918 (26), p. 424, Taf. xiv, figs. 17a, b. Reared at 

 Monaco. 



The left anterior coelom represented by a widened end of the 

 stone-canal, and a short wide pore-canal opening externally. 

 Hydrocoele provided with four lobes, but owing to the absence 

 of amniotic invagination its development was abnormal. One 

 of the primary tentacles gave out a small branch which corre- 

 sponds to one of the paired tentacles. 



(c) Pore-canal and madreporic vesicle doubled. 



[Case 25.] Strongylocentrotus lividus. Runn- 

 strom, 1918 (26), p. 419, Taf. xiii, fig. 11. Reared at Monaco. 



Both axial sinuses beset each with a pore-canal opening to 

 the exterior by a dorsal pore. A pair of vesicular organs lying 

 each near the pore-canal of each side were identified without 

 doubt by the author as madreporic vesicles. The one on 

 the left side acquired later a communication with the left axial 

 sinus, while the other on the right side began to degenerate. 



