MALACOZOA. 15 



of sense and locomotion are generally disposed sym- 

 metrically. The muscles are attached to the skin; and 

 it is by the alternate elongation and contraction of certain 

 parts that they crawl on the ground, swim in the water, 

 and lay hold of objects ; but, as their limbs are not 

 supported by bones or other solid parts, their motions are 

 in general very slow. They are never furnished with feet 

 arranged in series on each side of the body, as in the 

 Osteozoa and Entomozoa. 



The blood of the Malacozoa is white, bluish, or limpid. 

 The circulation is double : the heart, composed of a single 

 ventricle, receives from one or two auricles the blood 

 which comes from the respiratory organ, and propels it 

 into the arteries which distribute it through the body, 

 whence it returns to the branchiae. As some molluscous 

 animals live in the air, and others in the water, some 

 have lungs, or rather a pulmonic cavity, and others 

 branchiae. 



They all have a more or less convoluted alimentary 

 canal, open at both ends, but varying in structure. 

 Their mouth is sometimes furnished with horny jaws, 

 or with hard or cartilaginous parts, or is variously pro- 

 longed. The stomach is sometimes simple, sometimes 

 divided into several parts. The intestine is never sup- 

 ported by a mesentery ; but there is always a very large 

 liver, and most of the species have salivary glands. 



In some the sexes are separated, in others united. 

 They all produce eggs, which however are sometimes 

 deposited externally, sometimes hatched in the interior 

 of the animal, which is thus oviparous or ovo-viviparous. 

 In all cases the young have from the first nearly the form 

 which they present when mature. 



