196 



THE EYE IN EVOLUTION 



Solenogastre 



Nudibranch 



Pulmonate, 



Limnoea 



Nautilus 



from the ventral surface serving for locomotion, a dorsal or lateral 

 fold of the body-wall to form a mantle or pallium within which lie the 

 gills, and frequently a shell. As a general rule, two cephalic eyes 

 subserve the visual function, but these may be replaced by more 

 rudimentary organs in the dorsal region or around the margin of the 

 mantle or at the end of the tentacles or the siphons. Occasionally 

 eyes are lacking, in which case the skin has usually some sensitivity 

 to light. 



The large phylum of Molluscs is conveniently divided into six classes ; 

 three are relatively unimportant, sluggish in habit, and live in the mud or sand 

 of the sea-bottom — the shelled placophorans and scaphopods, and the worm- 

 like soLENOGASTRES. The remaining three classes contain an enormous number 

 of species of great variety — Gastropods, Lamellibranchs (Bivalves) and 

 Cephalopods. 



The GASTROPODS (" belly-footed ") constitute a very varied group comprising 

 some 40,000 species and include three main classes : 



(a) OPiSTHOBRANCHS : sea-hares, Pteropods (transparent marine plankton 

 forms), and the brilliantly coloured Nudibranchs or sea-slugs which have no 

 shell ; 



(h) PROSOBRANCHS, an enormous and varied grouj^ including sea-snails, 

 whelks, limpets, Heteropods, etc. ; 



(c) PULMONATES. The abundant and universally distributed fresh-water 

 and terrestrial snails and slugs. 



The BIVALVES : shell -fish such as cockles, mvxssels, clams, scallops and 

 oysters which live within a rigid hinged shell often at the bottom of the sea. 

 They comprise some 11,000 species. 



The remaining class, the cephalopods, are the 

 most interesting ; they are usually active, moving by 

 jet propulsion with a jet of water expelled from the 

 siphon. Two orders are recognized : the Tetra- 

 branchiates, with two pairs of gills, represented by a 

 single living species, the Pearly Nautilus of the South 

 Pacific, and the Dibranchiates, with a single pair of 

 gills and remarkably well-developed eyes (cuttlefish, 

 sqviid, octojxis). 



In the most primitive type of molluscs, the 

 PLACOPHORANS, cycs may be lacking althovigh some of 

 their sensory organs may be sensitive to light (Plate, 

 1899; Nowikoff, 1907). Some of them possess a multi- 

 tude of minute ocelli ; Corephiutn, for example, may 

 have as many as 8,500. The most interesting in this 

 class are the Chitons (" coats -of-mail") ; these possess 

 cephalic eyes in the larval stage which, however, dis- 

 appear as the advilt becomes clothed by its eight- 

 plated dorsal shell, thus rendering them useless. In 

 the- ■ place numerous innervated papillis appear con- 

 tai: i.ij sensory organs {aesthetes) which perforate the 

 shei. :^)earing in rows as minute black dots (" shell- 

 eyes loseley, 1884) (Fig. 183). The larger of these 



Fig. 



183. — The Mollusc, 

 Chitox. 



The sense organs, aes- 

 thetes, perforate the shell, 

 appearing as minute black 

 dots ; the larger of these 

 contain an ocellus (Thom- 

 son's Zoology, James 

 Ritchie ; Oxford Univer- 

 sity Press). 



