SYNAPTA VIVIPARA. 71 



the coelomic epithelium pass to form the blood corpuscles. Theoretically the vessels 

 ought to be lined with connective tissue of mesenchyme cells but these are so few in the 

 early stages of the larva, that the connective tissue between the laminae of the mesen- 

 teries or even around the digestive tract is very hard to demonstrate. The main blood 

 vessels appear to be purely entodermal in origin and their walls seem to be made up 

 solely of the coelomic wall. At any rate, the dorsal vessel does not arise in S. vivipara 

 iu the way given by Semon ('88) for its origin in S. digitata, by a simple split in the 

 mesenchyme where the two coelomic folds unite above the intestine to form the 

 mesentery. 



Very soon after the pentactula stage is reached the first rudiment of the genital 

 system appears. It arises on the inner side of the right-hand lamina of the dorsal 

 mesentery between the stone-canal and the oesophagus. The first appearance is simply 

 the increased size of the cells at this point, resulting in a thickening of the wall (Fig. 

 32), but the cells soon multiply rapidly and form a more or less spherical mass within 

 the mesentery (Fig. 33). As this mass increases in size, cavities appear within it 

 (Fig. 34) and these increase in size and begin to unite together until they form one 

 central lumen for the gland (Fig. 35). Meanwhile the right lamina of the mesentery 

 forms an outer epithelium which soon becomes quite distinct from the central mass of 

 cells. Between this epithelium and the remainder of the gland a few mesenchyme cells 

 form an extremely scanty connective-tissue layer. In the full-grown ten-tentaculed 

 larva, the genital gland is plainly seen (Fig. 19), lying entirely on the right-hand side 

 of the mesentery, near the stone-canal. 



The ten-tentacled larva (Fig. 19) is a more clearly defined stage in the development 

 of Synapta vivipara than is the pentacula; that is, it seems to last longer, and the rela- 

 tive condition of development of the various organs is more constant. The changes 

 which occur subsequently in the assumption of the adult form must now be considered. 

 The most obvious of these is the increase in the number of tentacles which arises from 

 the addition of another tentacle to the right and left dorsal interradii (Fig. 86). The 

 extra tentacle of the left side arises from the left dorsal " secondary outgrowth " of the 

 hydrocoel ring, which has hitherto remained in its original position beneath the left 

 dorsal nerve but now pushes out on its lower or ventral side and forms the eleventh ten- 

 tacle. At the same time, the extra tentacle of the right side arises from the lower or 

 ventral side of the right dorsal " secondary outgrowth " which pushes out on that side 

 of the right dorsal nerve and forms the twelfth tentacle. Not infrequently individuals 

 are found with thirteen tentacles, and in these the extra tentacle is usually in one of the 

 ventral interradii. In such cases it is probable that the mid-ventral " secondary out- 



