CHAPTER 12 



THE EYES OF MOLLUSCS 



Having considered the geological evidence of the existence of median and 

 lateral eyes afforded by the extinct Order of the Trilobites, which may 

 be regarded as the precursors of certain living representatives of the 

 Arachnida and Crustacea and which also have affinities with the Eury- 

 pteridae, an extinct Order of the Arachnida and the Xiphosura, we will turn 

 our attention to the Mollusca, in which phylum we meet with a great variety 

 of types of eyes, from the simple epithelial pit (limpet) or simple camera type 

 of eye without lens, as in Nautilus, to highly differentiated organs such as 

 are found in the cuttlefish, squids, and octopus. In many of the Mollusca 

 there is a hard shell which can be preserved in a fossil state ; and since 

 many of the fossil shells resemble those of living species, they give a clear 

 indication of the type of animal which was enclosed within the shell, 

 notwithstanding the fact that many of these fossil shells have been found 

 in Palaeozoic strata as far back as the Cambrian period. Moreover, it 

 is significant that living species possessing simpler types of eye, such as 

 the limpet (Patella) and the pearly Nautilus, are represented by fossil 

 shells found in the older Palaeozoic strata, while the more highly evolved 

 types of Mollusca, such as Sepia and the octopuses, having complex eyes, 

 are represented by fossil relics which are found only in the more recent 

 periods. 



In the Class Pelecypoda (Lamellibranchiata), which includes the 

 " right and left " bivalved shell-fish such as the mussels, cockles, oysters, 

 and scallops, the sense organs are not highly developed. This is due 

 to the fixed or non-motile condition of the adult animal, which usually 

 lies on one side at the bottom of the water. In some, e.g. Pecten (Fig. 

 1 06), marginal eyes of a highly differentiated type are found round the 

 edge of the mantle (Fig. 107). Nevertheless, whether eyes are present 

 in the adult animal or not, their free-swimming larvae pass through 

 trochophore and veliger stages in which an apical plate, cerebral ganglia, 

 cerebral pit, and statocyst are developed (Figs. 108 and 109). These 

 resemble in their essential characters the veliger larvae of other Classes of 

 Mollusca, such as the Amphineura, which includes the Chitons — " coat- 

 of-mail shells " — and the Class Gasteropoda, comprising the whelks, 

 snails, and limpets, in which paired eyes of a simple upright type are 

 found both in the adult and larval form of the animal (Figs, no, ill). 



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