5 i6 



THE POPULAR SCIENCE MONTHLY. 



Still, the chalky deposits are not wanting in them, but they are 

 microscopically small, are scattered, and rarely exhibit patterns 

 or knobs. The radial structure is easy to recognize in many of 

 them, but not in others, especially in the deep-sea forms which 

 have only recently become known. But this is a modern varia- 

 tion. The ancient typical 



,i . '■"■ ":' >!'.' 



«kz 





Fig. 2. — Sea Cucumber. 



structure of all the spiny- 

 skins is radial, and the 

 number of rays is five or 

 a multiple of five. When 

 rays appear in other nu- 

 merical relations (based on 

 1, 4, or 6), it may be traced 

 back to a recent variation. 

 While the mouth in the 

 sea urchins and starfishes 

 is on one of the broad sides 

 of the somewhat flattened 

 body — which for this rea- 

 son is designated as the 

 buccal, or, not very accu- 

 rately, ventral region — in 

 the sea cucumbers the body 

 extends from the mouth to the other pole, and the animals are 

 not flat, like their relatives, but lengthened out like worms. They, 

 therefore, do not move on the mouth-surface. Thus they are 

 transformed from a radial symmetrical structure into an appar- 

 ently bilaterally symmetrical, right-and-left structure, but really 

 equally lateral, and, superficially regarded, look like thick, plump 

 worms. Around the mouth is a fringe of tentacles, shield-formed 

 or greatly branched, which serve as organs of touch and groping, 

 or perhaps for breathing. The size of the animals varies greatly ; 

 there are forms in the depths of the northern seas which measure 

 but little more than a few centimetres, while tropical species living 

 near the surface are two feet long and more. Their stupidity and 

 slowness of motion correspond with the kind of food they live on. 

 They fill themselves with sand and the detritus of crumbled 

 corals ; and, as they do not hunt for food, they need no eyes or or- 

 gans for rapid motion. In those sediments of the sea are enough 

 organic substances — products of decay, algae, animals of the lowest 

 species — to keep the slow metabolism in action by their motion. 

 Such inert animals would soon fall a prey to the always hungry 

 robbers of the sea if they had to depend on their skill and dex- 

 terity. But they seem to have other means of keeping their ene- 

 mies away ; possibly they have a bad taste to them, or their 

 tough, leatherish skin causes them to appear undesirable morsels 



