Development of Ilolotliurians. 421 



lutely constant ]iosition in wliicli IK'ioiiard also met with it 

 in the adult. Ko valvular arningement whatever is present 

 at its widc-mouthrd o|)enin<i; into the water-vascular ring. 

 From the fifteenth day onwards circular muscle-fibres may be 

 recognized in its wall : they arc arranged concentrically in a 

 single layer round a point corresponding to the blind end ot" 

 the vesicle. The muscular layer ceases at the opening of the 

 vesicle into the water-vascular ring. In its origin it also is 

 derived from the hydrococle-cells, which represent the inner 

 e])ithclium of the entire water-vascular system. 



The young stone- canal possesses a vesicle-shaped expan- 

 sion, overlooked by Selenka, tlie epithelial coat of which 

 preserves the same constitution as in the rest of the stone- 

 canal only in the inner half of the vesicle («'. e. the one which 

 is turned towards the interior of the body), while in the outer 

 half {i. €, that lying nearer the surface of the body) it is 

 greatly flattened. I'iiis expansion is the earliest rudiment of 

 the subsequent madreporic head of the perfect stone-canal, 

 and may therefore be designated as the " madreporic vesicle.^^ 

 Hitherto it has only been casually noticed by Bury, and 

 termed by him the " anterior enterocoele." On the part of 

 the mcsenchyma it is surrounded by an incomplete calcareous 

 latticc-icorh envelope^ which has long been observed in other 

 llolothurians. The valve which was supposed by Herouard 

 to exist in the adult Cucnmaria at the exit of the stone-canal 

 from the water- vascular ring is not present; the columnar 

 epithelium of the stone-canal passes at this point almost 

 suddenly into the pavement- epithelium of the water- vascular 

 ring. The outer end of the primary stone-canal, leading from 

 the madreporic vesicle to the dorsal pore, lies, as does the 

 dorsal pore itself, which is subsequently obliterated, about the 

 eighteenth to twenty-fourth day, not in the median plane of 

 tiie Idolothurian as determined by the dorsal mesentery, but 

 to the riglit of it, wliich is once more explained by the oblique 

 position of this median plane with reference to that of the 

 larva, to which frequent allusion has already been made. In 

 the same way it is perhaps possible to explain the preference 

 which the stone-canal of adult Holothurians, especially in the 

 Aspidochirotffi, exhibits for the right half of the body. In 

 young animals of the ninety-eighth day the madreporic vesicle 

 has opened into the body-cavity on its thin-walled side, 

 thereby effecting the permanent connexion between the stone- 

 canal and tlie body-cavity. 



Nervous System. — On the eighth day of development rudi- 

 ments of the central portions of the nervous system, the 

 circumoral ring, and the radiol nerves already exist. Both 



