ORGANS OF THE SENSES. 1?59 



than in the active articulated classes, and they are never 

 aggregated together to form groups of simple eyes, like those 

 of myriapods or arachnida, nor compound organs like those 

 of insects and Crustacea. Where they occur in the acephalous 

 mollusca they are numerous, simple, and separate as in worms, 

 but in the higher forms of gasteropods, pteropods and 

 cephalopods their structure is more complex and their num- 

 ber is reduced to two, which are symmetrically disposed 

 on the sides of the head, as in all the vertebrated classes. 

 The tunicated animals, like the cirrhopods in their adult 

 state and like most of the entozoa, fixed and buried under 

 an opaque covering, appear to be destitute of visual organs, 

 and for the same reason these organs are as little required 

 and are wanting in most of the inhabitants of bivalve shells. 

 In the free and quick moving pectens, however, which swim 

 rapidly backwards by the quick and repeated contractions of 

 their valves, there are numerous distinct pedunculated eyes, 

 placed at the bases of the palleal tentacula all around the 

 free margins of the mantle, and these have long been figured 

 and recognised by naturalists as organs of vision common to 

 many of the conchiferous mollusca. They are placed in the 

 most exposed and the most sensitive part of the animal, and 

 by their pedunculated character and their position beside the 

 tentacula they resemble the eyes of gasteropods. They are 

 nearly a quarter of a line in diameter and more than fifty in 

 number in the pecten maximus. They have a round promi- 

 nent smooth cornea and contain an opaque shining choroid 

 embracing a small crystalline lens. Their nerves are probably 

 as in the gasteropods, derived from the tentacular branches 

 passing along the bases of their peduncles. Their forms, dis- 

 tribution and structure are the same in the spondyli as in the 

 pectens, and their shining lustre, in both these genera, is 

 compared to that of the emerald by Poli who has given en- 

 larged views of their microscopic structure. The eyes of the 

 gasteropods are always two in number, placed on the ante- 

 rior part of the body, moveable, and generally pedunculated. 

 Some of the naked dorsibranchiate gasteropods as the eolis, 

 the doris, and the glaucus appear to be destitute of these 

 organs. In the naked aplysia they appear as minute black 

 points on the smooth surface of the neck. In the pectini- 

 branchiate species they are most frequently placed on tuber- 



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