XII 



PHYLUM MOLLUSCA 



705 



muscle (Fig. 621, col. m.) which extends from the concave right 

 side of the animal to the columella, into which it is inserted ; it 

 is by means of this muscle that the anterior portion of the body, 

 capable of being thrust out through the mouth of the^shell, may 

 again be withdrawn. 



If the Triton be examined in the extended condition (Fig. 621) 

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



liv 



tent 



FIG. 621. Lateral view of the body of a female specimen of Triton rubicundus, removed 

 from the shell, moderately extended, col. m. columellar muscle ; cott. collar of mantle ; 

 eye, eye ; liv. liver ; mant. cav. mantle-cavity ; meso. mesopodium ; op. operculum ; 

 ow. ovary ; prop, propodium ; *iph, siphon ; stom. stomach ; tent, tentacles. 



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 

 retracted. Bach 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 is feeding, an elongated cylin- 

 drical introvert (Fig. 622), comparable 

 to that of Sipunculus (p. 484), is ex- 

 tended forwards, bearing the mouth at 

 its anterior end ; at other times the 



introvert is Completely involuted within FIQ. 622. Diagram of the introvert 



J ^s m*4i,M.M :,-v i.-x*-, ,-,: + ,, ^1 :^ i ^^^ 



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 longi- 

 tudinally ; this is the penis. Running 

 back from its base is a narrow groove with prominent lips the 

 sperm-groove, continuous with that of the penis ; in the female these 

 parts are not represented. 



Foot. On the side of the body (ventral) which the animal 

 applies to the surface of the ground when it extends from the 



in-l.ro 



of Triton, in longitudinal sec- 

 tion, as it appears when almost 

 completely extended. The black 

 lines represent the wall of the 

 alimentary canal ; the cross- 

 hatched part the wall of the 

 introvert ; the dotted line marks 

 the position of the opening 

 through which the introvert 

 passes, intro. introvert ; mo. 

 mouth ; ces. lumen of oesophagus. 



