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GENERAL ZOOLOGY 



The Nudibranch. The strange-looking creature shown in 

 Fig. 115, Dendrono'tus arbor es'cens, is to be found on seaweeds 

 and under submerged rocks along the north Atlantic coast. 

 Tree-like processes grow on the upper part of the body. 

 These processes are to some extent used in respiration. 

 Since they are thin, oxygen can pass into the animal through 

 them, and the waste carbon dioxide can pass out. The group 

 of animals to which they belong is called Nudibranchia'ta, 



in reference to the fact 

 that their gills are naked. 

 There is no indication of 

 a rudimentary shell. 



Definition of Gasteropoda 



(Gr. gaster, "stomach"; 



pous (pod-), "foot"). 



The class represented by 



the nudibranch, the slug, 



and the snails is given 



the name Gasterop'oda. 



The class name is only figuratively correct. The ventral 



surface, not the stomach, is modified to form a locomotor 



organ, the foot. 



An important characteristic of the Gasteropoda is the un- 

 symmetrical arrangement of organs. With the exception 

 of the mouth and the opening of the mucus gland, all the 

 openings of the body are on the right side, even in cases like 

 the slug and nudibranch, where the general form of the 

 body is bilaterally symmetrical. When a shell occurs it is 

 composed of one piece, and the characteristic form is spiral. 

 A shell-forming organ, the mantle, is usually present. 



The body of a gasteropod is not divided into somites, but 

 the head is slightly marked off from the rest of the animal, and 

 is provided with eyes and nonjointed tentacles. A tongue- 

 like organ bears a ribbon of minute teeth for tearing food. 



Fig. 115. Photograph of living nudi- 

 branch. (Natural size) 



