SITUS INVEKSUS IN ECHINOIDS 129 



at first as a solid thickened end of a string of cells given out 

 from the posterior end of this coelom. Later (16, pj), 261-2) 

 he confirmed this in Echinocar dium cor da turn, in 

 which species the vesicle in question is unusually large. 

 K u n n s t r o m found a pair of madreporic vesicles in a larva of 

 Strongylocentrotus livid us (Case 25), and, moreover, 

 according to him, the one on the left side became later connected 

 with the axial sinus of the same side. Perhaps other instances 

 of the presence of a communication between the vesicle and 

 one of the axial sinuses (Cases B, 1, 2, 16) may also be due to 

 a secondary change. In the Case 2 the vesicle is seen later 

 again separated from the coelom. Often this vesicle is absent 

 (Cases 14, 20). v. Ubisch (30, p. 443) is of the opinion that 

 the madreporic vesicle was not possessed by the ancestor of 

 the sea-urchins, but that it represented the only remnant of the 

 degenerated right anterior coelom having assumed a new 

 but unknown function in the course of phylogenetic develop- 

 ment. And, further, according to him, when the right anterior 

 coelom made its unusual development the highly-differentiated 

 and functioning madreporic vesicle could not be affected 

 thereby and both of them existed side by side. 



In the reversed and also in some double-hydrocoele larvae 

 (Cases 10, 11) the madreporic vesicle was found on the right 

 side, close to the right pore-canal. In the case where two such 

 vesicles are present (Case 25) the right one may be the homo- 

 logue of this. In neither case is its origin made clear. From 

 want of sufficient material and from our ignorance of its func- 

 tion any definite statement will be premature. 



(c) Pore-canal and Dorsal Por e. — The primary dorsal 

 pore is formed from the left coelomic sac to communicate with 

 the exterior before the latter becomes divided into the anterior 

 and posterior coeloms. In the course of further development 

 of the larva the pore shifts from its original position on the left 

 side towards the mid-dorsal line. This shifting is preceded by the 

 formation of a transverse groove of the ectoderm. Probably 

 in connexion with this shifting process it is often the case that 

 the canal gets temporarily or permanently obliterated (Cases B, 



