26 Echinoderma. VI. Holothurioidea. 



visceral layer, but parietally are confined to five bands which are not developed 

 from the water- vessels, as stated by Selenka [see Bericht for 1883 I p 127]. 

 The second set of plates in the oral calcareous ring are situated like the first in 

 the spaces between the tentacles, i. e. interradial ; but the water-vessels main- 

 tain their first relation to the 5 primitive pieces and so become adradially dis- 

 placed, though the stone-canal remains interradial. The reduced oral funnel 

 expands into the atrium and again opens externally, for the protrusion of the 

 tentacles. These being used for locomotion, the ciliated bauds disappear, their 

 grooved mesenchym supports being flattened out, while the ciliated cells give 

 rise to the ordinary and sensory cells of the epidermis. Towards the end of the pupa 

 stage the madreporite appears in the length of the stone-canal, so that it opens 

 into the coelom, while its distal end and the water-pore are obliterated. Cor- 

 puscles then appear in the tentacles and water- vessels which are probably respi- 

 ratory, and derived from their lining epithelium ; while those of the coelom are 

 formed by the peritoneal epithelium. The mesenchym becomes compacted, the 

 processes of its cells developing into connective-tissue fibrils; while schizocoel 

 spaces appear in it, those on the dorsal and ventral sides of the gut becoming the 

 blood-lacunae, but none develope in the body wall. - - The Synaptidae cannot be 

 regarded as degenerate. Their mode of life is not sufficiently subterranean to 

 make them so, while the nervous system and sense organs are too highly deve- 

 loped. There are no embryological grounds for so considering them; neither are 

 they sexually mature larvae. 





