1 LLUSTR \ TED N ATT K A L II [STORY 



crrTi.E-risu : DIVISION MOLLUSCA 



Some of the lower forms, arranged with the molluscous animals by modern zoologists, possess 

 only a single ganglion, from which filaments are given off in ail directions; and between this and 

 the highly complicated structure, we meet with every conceivable gradation. 



A- mighl be expected from the great differences displayed by the members of this division 

 of the animal kingdom, in regard to the degree of development of the nervous system, the 

 • - as I by them in very various degrees of perfection. In swine of the lowest forma 

 the universal sense of touch appears to be the only one present ; but as we ascend in the scale, wb 

 meet with creatures more highly endowed in this respect Tentacles, or special organs of touch, 

 Frequently o . g< m rally in the neighborhood of the head; organs o( sight, hearing, smell, and 

 ■ make their appearance, until in the highest forms of molluscous animals we rind the organs 

 of the senses 3 fully d( 1 a- in many of those belonging to the highest division. 



The skin of these animal.- generally lies loosely about the body, SO as to form a sort of cloak or 

 tie. The mantle frequently p< - &s - the power of secreting a hard substance, well known a- 

 thc shell, which - for the protection of the creature. It increases with the growth of the ani- 



mal, and varies in form ding t<> the s] - which inhabits it. 



The intestinal canal is very variable in its structure, but always presents two openings — a mouth 

 and an anus; the liver frequently attains a very great degree of development. 



The circulatory system is generally very highly organized; a heart, often divided into several 

 compartments, with arteries and veins penetrating all parts of the body, existing in nearly all the 

 Mollusca. The bl 1 is colorless, or nearly so. 



Th<- Mollusca are oviparous animals; the male and female organs are frequently in separate in- 

 dividual-, although many sp< cies are hermaphrodite. 



In the fifth and highest division of the animal kingdom, we meet with ;i series of organs 

 which nothing similar occurs in the group- which have already passed under review. 



All these animal- possess a nervous Bystem, consisting essentially of a brain, inclosed within a 

 bony case, the skull, beneath which the oesophagus passes, and a single cord of nervous matter, 



