NATURE OF THE INVERTEBRATE BRAIN. 



33 



far the greater number are aquatic, and breathe by means of gills. 

 But being all of them, as Prof. Owen says, "endowed with power to 

 attain, subdue, and devour organic matter, dead and living," we find 

 their nervous system not only better developed, more complex and 

 concentrated, but also in relation with more highly-evolved organs of 

 special sense and exploration. It offers considerable variations in its 

 general arrangement, especially as regards relative positions of gan- 

 glia, though these modifications are, to a great extent, referable to 

 differences in the outward configuration of the body. 



Some of the differences in external form which are to be met with 

 among gasteropods are well illustrated by the limpet or the chiton, as 

 compared with the snail. Here differences in form coexist with dif- 

 ferences in habit, so that we almost necessarily meet with notable 

 variations in the disposition of the principal parts of the nervous 

 system. 



In the limpet we find that the two small cerebral ganglia (Fig. 

 10, a) are widely separated from one another, and lie at the side of the 

 cesophagus. Each receives a rather large nerve from one of the ten- 

 tacles, and a smaller optic nerve. A commissure connects these cere- 

 bral ganglia above the cesophagus with one another, while each of 

 them is also in relation by means of two descending commissures 

 with a series of four connected ganglia forming a transversely-ar- 

 ranged row beneath the cesophagus. Of these the two median gan- 

 glia (JB) correspond with the pedal, while the two external ( C) cor- 

 respond with the branchial ganglia, though they are here separated 

 from one another by an immensely wide interval. 



Fig. 10. Nervous System of the 

 Common Limpet. 



Fig. 11. Nervous System of Chiton 

 marmoralus. 



However small and undeveloped t-he duplex brain of the limpet 

 may be, this organ exists in an even more rudimentary state in some 

 other gasteropods. Thus, in the chiton, which is a close ally of the 

 limpet, and about the most simply organized of all the gasteropods, 

 there are neither tentacles nor eyes, and, as a consequence of this, 



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