TYPE MOLLUSC A. 



301 



commissures, recalling the condition seen in the Chitons, as 

 well as the ladder-like arrangement of the ventral nerve-cords 

 of the Annelida, though there cannot in the Gasteropods be 

 any question of metamerism in this connection. 



Special sense-organs are very generally well developed in 

 the Gasteropods. The tentacles so usually found upon the 

 head have probably a tactile function as well as the tentacular 

 or winglike processes sometimes found in connection with 

 the anterior extremity of the foot, and the epipodial ten- 

 tacles which occur in some forms (Haliotis). On the ventral 

 side of the bases of the epipodial tentacles of some forms 

 special sensory thickenings have been found which have 

 suggested a comparison with the sense-organs of the lateral 

 line of the Annelida, a comparison which, however, at present 

 seems rather strained ; it seems probable, notwithstanding 

 their inuervation from the pedal ganglia, that these sensory 

 patches are to be placed in the same category as the osphra- 

 dia and the sensory ridges of the mantle-cavity of the Chi- 

 tons. The osphradia (Fig. 133, os) in all Gasteropods which 

 are provided with branchiae are associated with these organs ; 

 and even where one or both 

 branchiae have been suppressed 

 the osphradia may still persist. 

 Eyes (Fig. 134) are very generally 

 present in the Gasteropods, being 

 situated at the base of the ten- 

 tacles, or at their summit in some 

 forms. They present a very uni- 

 form structure throughout the 

 group and arise as a depression 

 of the integument, the lips of 

 the cavity fusing and giving rise 

 to a globular sac lying beneath 

 the epidermis, which remains thin and transparent, forming an 

 outer cornea (co). The cells of the outer wall of the sac 

 likewise remain clear, forming the inner cornea, while over 

 the remainder of the wall of the sac they are sensory in 

 function, piginented cells being scattered among them, the 

 two together forming the retina (ret). The nerve-fibres pass- 



FIG. 134. EYE OF Hnliotis (after 

 PATTEN). 



co = cornea. 

 I = lens. 

 rt = retina. 



