STRUCTURAL MODIFICATIONS 



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lessly in the sand, and in these the lower region of the body be- 

 comes flattened to form a ventral side, while the mouth advances 

 gradually towards the edge of this surface that the animal keeps 

 habitually in front. Above this, one of the ambulacral areas, 

 which has become the anterior one, moves up to the top 

 of the body, round which the other four ambulacral areas, 

 now become lateral, continue to converge ; the anus leaves 

 the top for a definitely posterior position, and finally reaches 

 the region of the edge of the flattened lower surface, which 

 thus forms a ventral side situated between the two lateral 

 ambulacral areas that are furthest away from the anterior one. 

 The Holothurians, or sea-cucumbers are Echinoderms closely 

 akin to the Sea-urchins, but the body is elongated like a 

 sausage instead of being globular. They often live in the crevices 

 of rocks ; they can only maintain themselves on the ground in 

 a recumbent posture, the openings of the digestive tube each 

 occupying one end of the body. A certain number of littoral 

 species, however, crawl on the sea-bottom ; they then acquire a 

 flattened ventral side always divided by one of the radial loco- 

 motor areas into two symmetrical halves and bounded by two 

 lateral ones. Exactly the reverse condition is found among the 

 Sea-urchins, where the ventral side has no median radial area 

 while the dorsal side has. The mechanism of the formation of 

 the ventral side, however, is quite different in both cases. The 

 Holothurians with ventral sides behave in two ways. Those 

 inhabiting great depths live on mud ; they direct the mouth 

 towards the earth by sharply bending the anterior extremity 

 of the body. At first temporary, this bending becomes later 

 permanent, and subsequently disappeared, the mouth finally 

 becoming definitely ventral. Those attached to rocks, on the 

 contrary, draw their nourishment from the surrounding water, 

 and their mouth becomes dorsal (Psoitis). Moreover, certain 

 Holothurians always live buried in sand ; some remain 

 elongated vertically, 1 while others curve their body into a 

 U so as to bring their anus to the surface and evacuate their 

 excrement without soiling the sand. This attitude, at first 

 temporary, 2 also becomes fixed, 3 so that the two ends of the body 



1 Molpadia, Ankyroderma, Synapta. 



2 Certain Cucumarics. 



3 Ypsiloth aria. 



