2 78 ZOOLOGY FOR MEDICAL STUDENTS chap. 



the great majority of gasteropods. Euthyneury is found in the Pul- 

 monata in which the loop is too short to be twisted and it is also found 

 in an interesting group of gasteropods — the Opisthobranchiata — in which 

 the pallial complex has become shifted backwards again along the right 

 side of the body so that the loop has been untwisted. 



While the above-described condition "of the nervous system with its 

 distinct ganglia is found in the majority of existing gasteropods it is 

 probably not primitive. In the Mollusca as in other groups the evolution 

 of the nervous system has been marked by gradually increasing con- 

 centration from a primitive more diffuse condition, and in fact when we 

 examine the most primitive gasteropods such as Chiton we find the 

 central nervous system consisting of broad strands with gangUon cells 

 scattered over their surface instead of being concentrated into definite 

 ganglia. In the Pelecypoda (Fig. 114, C) there is usually a set of ganglia 

 resembling that of a Euthyneurous gasteropod except that the cerebral 

 and pleural ganglion on each side are fused into a single ganglionic mass. 

 Of the Siphonopods the most primitive, Nautilus, has a nervous system 

 of broad strands like those of Chiton, while the others have their ganglion 

 cells concentrated into rounded ganglionic masses resembling somewhat 

 though less distinct than those of Gasteropods. 



The molluscs have, except in the region of the shell, soft sensitive 

 skins with scattered sensory cells. In addition they have masses of 

 sensory cells concentrated together to form definite sense organs — eyes, 

 otocysts, osphradia. Of these the eyes are of special interest, for 

 whereas some members of the phylum — the Cuttlefishes — possess eyes 

 which are amongst the most complicated and most highly developed 

 known, others such as the ordinary Limpet {Patella) possess eyes in 

 the very earliest stages of evolution. Either the comparative study 

 of the structure of the eye in the adults of different molluscs, or on 

 the other hand the study of the various stages in the embryonic de- 

 velopment of one of the more complicated eyes like that of the 

 Cuttlefish, serves to give us a wonderfully clear, and complete picture 

 of how these organs have gradually become perfected in the course of 

 evolution. 



In the Limpet (Fig. 115, A, i), the eye consists of a localized thickening 

 of the epidermis (the retina) containing numerous slender sensory cells 

 sensitive to light and continued at their inner ends into nerve fibres : 

 this sensory thickening of the epidermis dips down below the surface 

 in the form of an open pocket. In the " Silver Willie " {Trochus — 

 Fig. 115, A, 2) the eye shows a slight evolutionary advance, the pocket 

 being in this case distended to form a hollow vesicle, with a narrow opening 



