R ADI ARIES. 195 



aries. Lamarck has divided this class into two 

 orders, the Gelatines* and the Ecliinoderms^ 



1. The Gelatines^ which some consider as a 

 distinct class under the name of Acalephes? are 

 distinguished by a gelatinous body, and a soft 

 and transparent skin ; they have no retractile 

 tubes issuing from the body ; no anal passage ; 

 no hard parts in the mouth ; and they have no 

 interior cavity, their viscera being imbedded in 

 their gelatinous substance. 



Some genera 4 in this Order, like the fishes, 

 are remarkable for an air-vessel which they 

 can fill or empty, and so rise to the surface, or 

 sink to the bottom at their pleasure, but it differs 

 from that of the fishes in being external ; others 

 are distinguished by a dorsal crest, which they 

 erect and use as a sail. 5 



2. The Echinoderms have an opaque, leathery, 

 or crustaceous skin, mostly covered with tuber- 

 cles, or even moveable spines, and generally 

 pierced with holes, disposed in rows ; retractile 

 tubes which respire the water, and are used also 

 for locomotion and prehension, emerge from 

 these holes ; a mouth generally situated below, 

 and armed with hard parts ; and a cavity simple 

 or divided. 



To begin with the Gelatines in walking upon 



1 Radiaires molasses. z R. Echinodermes. 3 Acalepka. 

 * Physsophora. &c. 5 Vellela. 



