ORGANS OF SENSE. 57 



scribed in many of the polygastric animalcules ; they vary in number 

 from one in the polygastrics, up to eight in the acalepha. In the 

 rotifera, optic nerves and ganglia are quite visible, and in the aca- 

 lepha a lenticular body is superadded. Visual organs are met with 

 in almost every class of the diplo-neurose division ; in some there 

 is but a single eye, in others they are more numerous, and placed 

 apart on different aspects of the head. The medicinal leech pos- 

 sesses ten prominent eyes disposed transversely, and the neiris nun- 

 tia has two large pairs placed on the upper part of the head, and 

 nearly a hundred smaller ones grouped around the mouth. Many 

 of the higher insects present in their optical apparatus all the essen- 

 tial ingredients found in the highest forms of the organ. The eyes 

 of the crustaceans, are generally compound like those of insects, and 

 in many instances are moved by distinct muscles, and covered in 

 front by a transparent layer of epidermis. In the Crustacea, as in 

 insects, the optic nerves enlarge into a ganglion in the globe of the 

 compound eye, from which small filaments radiate to the several 

 lenses of the component eyes. 



Organs of vision are less required, and consequently less deve- 

 loped in the torpid mollusca than in the active articulata ; they 

 never form groups of simple eyes like the myriapods, nor compound 

 organs like the insects and Crustacea. In the acephalous mollusca 

 they are simple, separate and numerous as in worms, but in the 

 higher forms of gasteropods, pteropods, and so on, they are more 

 complicated, and but two in number, disposed on the sides of the 

 head as in the vertebrata ; and in these possessing opaque coverings, 

 as the inhabitants of bivalve shells, they are altogether wanting. In 

 those, however, which enjoy rapid motion, as the pectem maximus, 

 they are upwards of fifty in number, each being about a quarter of 

 a line in diameter. The eyes of these animals are generally flat- 

 tened in front, they possess a rudiment of membrana nicitans; the 

 iris and the lids are usually motionless, yet in the general plan of 

 their formation they form a near approach to the condition of these 

 organs in the higher vertebrated classes. 



In all the vertebrata the eyes are two in number, and with the 

 exception of a few species, are symmetrically disposed on the sides 

 of the head ; the differences they present being chiefly referable to 

 the density of the media in which the various animals reside. From 

 the density of the watery element through which fish move, their 

 eyes are generally of considerable size, except in the worm-shaped 

 fishes, as the eel and the lamprey. Sometimes they are directed 

 backwards or upwards as in the star-gazer : less frequently they are 

 placed on one side, as in the sole. The eye is spherical posteriorly, 

 and flattened in front. The conjunctiva is continued across in front 

 of the cornea, and admits of easy separation from it in the eel and 

 many other species. The eyelids are merely rudimentary and the 

 lachrymal gland is wholly absent. The sclerotic tunic is thick, 

 deuse, laminated, elastic, and occasionally ossified anteriorly. The 

 choroid is divisible into three layers, the internal or tunica Ruys- 



