SYNAPTA VIVIPARA. 67 



connected with the exterior only by a very narrow canal (Figs. 42 and 43, and Fig. 17, 

 Igo.). These organs appear, from their structure and the great variation which they show 

 in staining, to be of a glandular nature, and I am inclined to think they maybe connected 

 with the absorption of nourishment from the fluid of the body-cavity of the mother, I'm- 

 they never increase in size, are most abundant in the young with ten tentacles, and seem 

 to have entirely disappeared in the adult, and finally, nothing of the kind has been described 

 for any other holothurian. When fully grown they measure about fifty mikrons in diam- 

 eter and somewhat less in depth. They consist of very long, clear cells, with nuclei at 

 the extreme distal ends, surrounding a more or less spacious lumen which opens to the 

 exterior by a narrow canal of ordinary epithelial cells. Sometimes the clear cells stained 

 heavily, but often they did not stain at all. 



Up to this time the mesenchyme cells have played no part in the development of the 

 larva. In Fig. 16, they are shown as they appear scattered almost uniformly through the 

 segmentation cavity. Shortly after this, however, they begin to gather around the lower 

 and outer edge of the water-vascular ring, and by the time the pentactula stage is reached 

 they have begun the formation of the calcareous ring. Contrary to Semon's ('88) views 

 on S. digitata, and in accordance with Ludwig's ('91) observations on Cucumaria, I have 

 found no evidence at all of any mesenchymatous musculature on the oesophagus. The 

 first products of the mesenchyme cells to appear are five small straight rods between the 

 primary tentacles but outside and somewhat below the hydrocoel ring. Soon after these, 

 five more appear below the bases of the tentacles, so that there are now ten rods, five 

 radial and five interradial, and they continue in this position so long as there are only five 

 tentacles. I saw no evidence at all of any such shifting of position of the first five rods 

 as Semon ('88) records for S. digilata, nor could I consider the position of the second 

 series as agreeing at all with his description. Very soon after the appearance of the ten 

 rods, they fork at the ends and begin to branch very irregularly. As will be seen from 

 Fig. 46 a-i, the divisions occur at all sorts of angles and not only differ decidedly from 

 Semon's figures of the same rods in S. digitata but show no sign at all of following his 

 law ('87) for the formation of calcareous plates in Echinoderms. The much-branched 

 ends of the rods come into very close contact but evidently never mingle, for the plates 

 into which they develop are at all times easily separable and the line of division between 

 them is practically straight. Very soon after the appearance of the first five calcareous 

 rods, the radial nerves grow backward over them and before the completion of the pentac- 

 tula, run to the posterior end of the animal. 



We have now reached the complete pentactula form, a slightly older stage of which 

 appears in Fig. 18. The pentactula is about half a millimeter long and its characteristics 



