VI. Holothurioidea. 23 



Ludwig ^) gives some further details respecting the morphology of Rhopalodina. 

 At the transition from the globular part of the body into the narrow neck the 

 spines on the plates of the calcareous armature are bent downwards towards the 

 lower pole. The longitudinal muscular bands are single, not paired. Those of the 

 four lateral radii flatten themselves out somewhat, for a partial insertion at the 

 inferior pole, while that of the mid-ventral radius continues uniform throughout. 

 Nerves and water-vessels , like the muscles , bend upwards again at the aboral 

 pole. Round the rectum the five muscles are of equal size, but round the gullet 

 the dorsal pair are thicker than the other three ; while the circular musculature 

 of the body wall is continued on to the partition separating the oral and anal parts 

 of the neck. This is the remains of the much shortened mid-dorsal interambula- 

 crum. The five pairs of radial papillae in the anal calcareous ring are the forked 

 ends of the water-vessels, the walls of which have a calcareous armature. But 

 the interradial points of the ring are solid, and extend over the radii too. 



Edwards has studied the embryology of Mulleria Agassizii. Gastrulation is 

 followed by an abbreviated Auricularia stage within the egg, and the embryo is 

 liberated on the 6 th day with 5 oral tentacles and a rudimentary calcareous ske- 

 leton. The ventral surface becomes marked by the development of tube-feet, 

 first a posterior, then a median, and then an anterior one. Two more appear 

 before the paired ambulacral feet, of which by the 88 th day there may be 24. 

 The calcareous rods appear early, and some of them fuse together, to form two 

 anal valves running longitudinally to the body, which wave, fan-like, from side 

 to side with the cloacal respiratory movements. 



Topsent notices a male Cucumaria in which the free end of one of the genital 

 tubes bore a small reniform body covered with yellow ciliated cells and contain- 

 ing an obscure internal mass consisting of calcareous corpuscles. This may have 

 been destined to be detached and liberated externally. 



