SYNAPTA VIVIPARA. 69 



radius shows as yet little tendency to develop either way (Fig. 85) . Consequently we 

 now have a larval form with ten tentacles, two in each interradius. As the accessory 

 tentacles grow very rapidly they are soon equal in size to, and cannot be distinguished 

 from, the five primary tentacles. It is hard to decide positively which of the five accessory 

 tentacles develops first, for apparently they all begin to grow at about the same time. 

 In Chirodota rotifera, Ludwig ('81) found the first two accessory tentacles in the lateral 

 dorsal interradii, and he does not speak of finding any trace of additional tentacles in the 

 other interradii. I have not found any stage similar to that in S. vivipara, and I think 

 the five accessory tentacles appear at practically the same time. It is an important and 

 interesting fact, however, that the five accessory tentacles are formed in precisely the 

 same manner and from the same radii as the second series of five tentacles in Cucumaria, 

 (Ludwig, '91). It seems to me that this fact proves satisfactorily that the radial canals 

 in Synaptidae are homologous with those of the other holothurians or, more accurately, 

 the secondary outgrowths of the hydrocoel ring in the Synaptidae are homologous with 

 the five outgrowths of the hydrocoel ring in the true holothurians. In both cases, the 

 ten-tentacled young has one primary and one accessory tentacle in each interradius. 

 While this change is taking place in the number and arrangement of the tentacles, a cor- 

 responding change is going on in the calcareous ring. As the accessory tentacles push 

 out into the interradii, the calcareous rod which lies at the base of the primary tentacle 

 cornes to lie between it and the accessory tentacle. I could not see that this came about 

 by any actual movement of the rod itself, but was due simply to the increase of width 

 in each interradius. In the further growth of the calcareous ring, the interradial pieces 

 send up projections between the two tentacles (Fig. 46 h) and at the same time branch 

 and divide so rapidly and irregularly that they soon become plates, with straight sides 

 but pointed anteriorly and notched behind, made up of a very fine irregular network of 

 calcareous strands (Fig. 44). The radial plates develop in the same way but send up two 

 projections, one on each side of the radial nerve (Fig. 46 i). which finally fuse together 

 above it and thus form the perforated plates of the ring (Fig. 45). About the same 

 time, the mesenchyme cells lying between the ectoderm and the wall of the coelom 

 begin to gather in groups close to the ectoderm and there give rise to anchors 

 and anchor-plates so characteristic of the adult Synapta. The development of the cal- 

 careous bodies from a straight rod takes place as described by Semon ('87) for Synapta 

 inhaerans. While these deposits appear in the body-wall as far anteriorly as the base of 

 the tentacles, in the walls of the latter, lying parallel to the long axis, there appear 

 numerous rather long, more or less, knobbed rods (Fig. 48) similiar to those described 



