54 INTRODUCTION. 



more or less solid than the rest of the skin, which covers the 

 trunk, and in some even the head. In several orders, the sto- 

 mach, which is highly muscular, is provided with a cartilagi- 

 nous skeleton and with tubercles or teeth. The intestinal 

 canal is commonly short and straight. The position of the 

 genital organs varies; in some genera these organs are double. 

 The eyes offer much variety, arid in a few are wanting; in 

 others they are very nearly joined, and seemingly thrown into 

 one; some again have compound eyes, supported by amovea- 

 ble pedicle. Finally, in some of the Crustacea decapoda, there 

 are distinct organs for hearing. 



46. The mollusca form a division of the invertebrata, in 

 which we generally find a symmetrical or binary form, but 

 no articulations. Their stomachs are simple or multiple, some- 

 times furnished with hard parts, and their intestines variously 

 prolonged. The greater part have salivary glands, all a vo- 

 lumnious liver, and many peculiar secretions. Their circula- 

 tion is double, and there is always at least one fleshy ventricle 

 which is aortic, and receives the blood from the organs of re- 

 spiration and sends it back through the arteries of the body. 

 In those that have more than one ventricle, they are not unit- 

 ed in one single mass, but form several distinct hearts. The 

 blood is bluish. The organs of respiration are sufficiently di- 

 versified to enable some to respire air and others water. Ge- 

 neration also in them presents all its varieties; some not hav- 

 ing the sexes and producing living young ones without copu- 

 lation, and others being hermaphrodites with a reciprocal 

 copulation, while in a third the sexes are separated. The eggs 

 of the latter are simply enveloped with a viscous matter, and 

 others again have shells more or less hard. These animals 

 are very prolific and very tenacious of life. Their muscles 

 are attached to the interior of a soft and elastic skin, and their 

 movements are produced by parts that have no solid levers. 

 They are highly irritable. Their naked skin is covered with 

 a mucous fluid that oozes from it. Almost the whole of them 

 have a development of the skin which covers up the body 

 like a mantle, variously diversified, however, as to figure. This 

 mantle sometimes remains soft, but most frequently it hap- 



