ABERRANT ANIMALS-THE MOLLUSKS AND ECHINODERMS 275 



Fig. 12-27. A basket star (Gorgonacephalus articus) taken in 420 feet of water. The five principal arms are sub- 

 divided into a great many smaller branches. This is a ventral view. 



teresting adaptations. The sea cucumber 

 (Fig. 12-26), for example, is quite unlike 

 the starfish in its general appearance, al- 

 though its fundamental structure is similar. 

 These animals are like cucumbers with a 

 fringe of tentacles on one end. While their 

 habitats vary widely, a common species of 

 our Atlantic coastal waters lives in the mud, 

 just below low tide, exposing only its tenta- 

 cles above the muddy bottom. It will serve 

 as an example. 



The surface of the body seems to be 

 devoid of calcareous plates, but micro- 

 scopic examination reveals tiny plates em- 

 bedded in the soft tissue of the body wall. 

 The branched tentacles are located at the 

 anterior end, surrounding the mouth; the 

 anus is at the opposite end of the animal. 

 There are five rows of tube feet, the two 

 dorsal ones functioning as respiratory and 

 tactile organs, while the three remaining 

 rows en the ventral side aid in locomotion, 



much the same as those of the starfish. The 

 animal is able to crawl in worm-like fashion, 

 by contracting the rather heavy muscles 

 which make up the body wall. It feeds by 

 allowing the tentacles to become covered 

 with detritis from the muddy ocean bottom 

 and then pushing them, one at a time, into 

 the mouth. Organic material is separated 

 from the mud and carried into the digestive 

 tract where it is digested. 



Respiration is carried on by means of a 

 pair of respiratory trees which extend from 

 the lower end of the digestive tract anteri- 

 orly in the body cavity. Water is taken into 

 the cloaca through the anus and circulated 

 through these ramifying tubes. It is likely 

 that excretory wastes find their way to the 

 outside through these organs as well. 



The sea cucumber has a nervous system 

 equivalent to that of the starfish. The ani- 

 mal's sensitivity to light is easily demon- 

 strated. If it is suddenly placed in a bright 



