074 



ZOOLOGY 



SECT. 



FIG. 573. Operculum of 

 Triton nodiferus. 



External features of soft parts. The Triton is able to 

 extend itself to a considerable degree beyond the mouth of the 

 shell ; but a portion of the body always remains concealed in the 

 interior, even when the animal has ex- 

 tended itself to its utmost, the body being, 

 like that of the fresh- water Mussel (and 

 of nearly all the Mollusca), organically 

 connected with the shell. In Triton the 

 connection is by means of a str >i 

 the columellar muscle (Fig. 

 which extends from the coil] 



side of the animal to the columella, into which it is ii 

 is by means of this muscle that the anterior portion oi 

 capable of being thrust out through the mouth of the 

 again be withdrawn. 



If the Triton be examined in the extended condition (Fig. 574) 

 it will be found to present a distinct head, which bears dorsally 

 a pair of appendages the tentacles (tent) of a sub-cylindrical 

 shape, slightly compressed towards their bases, and narrowing 

 somewhat towards their free extremities; these are capable of 

 being extended and contracted, but not of being completely re- 

 tracted. Each bears on its outer side some little distance from 

 the base a prominent eye. At the anterior end of the head on 

 its ventral aspect is the opening of the mouth. When the animal 



liv 



siph 



tent 



rnesopcL 



propd 



;. 574. Lateral view of the body of a female specimen of Triton nodiferus, removed from 

 the shell, moderately extended, col. m. columellar muscle ; coll. collar of mantle ; lie. liver ; 

 mant. eav. mantle cavity ; meso. mesopodium ; neph. nephridium ; op. operculum ; ov. ovary ; 

 prop, propodium; stom. stomach; tent, tentacles. 



is feeding, an elongated cylindrical introvert (Fig. 575), comparable 

 to that of Sipunculus (p. 451), is extended forwards, bearing 

 the mouth at its anterior end; at other times the introvert is 

 completely involuted within the head and anterior portion of 

 the body. In the male on the right-hand side of the body, some 

 little distance behind the head, is a long narrow fleshy process, 

 broader at the base than at the free end, and deeply grooved 



