VIII EGHINODERMATA—ONTOGENY 519 



tissue layer of the intestine. In a similar way, the parietal wall of the enterocol 

 presses the peripheral mesencliyme cells to form a layer below the ectodermal body 

 epitheliiim ; this layer forms the cutis of the body wall. 



Whilst these processes have been going on (i.e. while the entorocccl vesicles are 

 increasing in size till they surround the intestine, the parietal wall becoming applied 

 to the body wall, and the visceral wall to the intestinal wall of the young animal, 

 and the two vesicles opening into one another ventrally), the narrow slit-like cavities 

 of the watch -glass -shaped enterooo-l vesicles in the larva have become the large 

 spacious body cavity. 



The visceral wall of the enterociel yields the intestinal musculature and the 

 endothelial covering of the intestine ; the parietal wall of the entei'occel gives rise 

 to the longitudinal and circular musculature of the body wall and its endothelial 

 covering. Since the musculature of all parts of the water vascular system is 

 formed from the epithelial wall of that system, it follows that the whole of the 

 musculature of the Holothurian body is of epithelial origin. 



The blood lacunar system arises in the form of cavities in the connective tissue 

 (mesenchymatous) layer of the different organs (integument, intestine). 



The rudiments of the genital organs, of the respiratory trees, and of the Cuvierian 

 organs are unknown. 



Not all the Holothurids pass through a distinctly marked Auricularia stage. 

 The gastnila larva of Cucmnaria Plaiici, for example, passes direct into the stage of 

 the barrel-shaped larva. Nevertheless, "when the oral depression of the Cucmnaria 

 begins to form, it is beset along its edge with wreath-like ectodermal protuberances 

 (ciliated protuberances) which, taken together, may be compared with the ciliated 

 ring of the Auricularia larva." One trace of the Auricularia stage seems thus to 

 be retained. 



The preoral region in the Cucmnaria larva rises up sharply as the frontal 

 prominence, and in its gelatinous nucleus an excessive growth and multiplication of 

 the mesenchyme cells early takes place. This is resorbed at a later stage as 

 nourishment for the formation or further development of the different organs. 



The plane of symmetry of the young Holothurid does not coincide with that of 

 the larva, but deviates from the latter, anteriorly to the left, and posteriorly to the 

 right. The long'itudinal axis of the young Holothurian deviates anteriorly ventrally, 

 and posteriorly dorsally, from that of the larva. 



C. Ontogeny of the Echinoidea. 



The following description is an epitome of the results gained by various investi- 

 gators in the examination of different Echinoids. 



Segmentation is total, and, in a peculiar manner, unequal. The inequality 

 between the blastomeres, however, soon almost entirely disappears. A spherical, 

 egg-shaped, or (Echinocyam-its pusillus) long elliptical cceloblastula arises, with a 

 unilaminar wall, which becomes covered with long flagellate hairs (one on each cell). 



The wall of the blastula becomes thickened at the vegetative pole. At this 

 thickened part, the blastoderm cells divide actively, so that the wall becomes bi- or 

 trilaminar. The deeper cells pass in succession into the segmentation cavity, 

 become anm-boid, and are the first mesenchyme cells. At this same point the 

 blastula wall sinks in to form the archenteron. The blastula becomes a gastrula. 

 During this process of invagination, the wandering of mesenchyme cells out of the 

 wall of the archenteron into the blastocrel continues. 



In Echinocyamvs (and other Echinoids ?) the cells of the blastula wall at the 

 animal pole (the point opposite the later blastopore) arc longer than the rest even at 



