INVERTEBRATA — MOLLUSCA. 753 



which the nervous system, composed of scattered masses, is contained in 

 this general envelope ; in which there is a complete circulating system, 

 particular organs for respiration, and organs for digestion and secretion, are 

 termed Mollusca. 2. The second division, including those animals in which 

 the trunk is divided transversely into a certain number of rings, and of 

 which the integuments, either hard or soft, have always the muscles attach- 

 ed to their interior, is named Articulata. The nervous system in this divi- 

 sion consists of long threads, running along the belly, and thickened at 

 certain distances into knots or ganglions ; and the body is, in most cases, 

 provided with jointed members, or legs, at the sides of the annular segments. 

 Their jaws, whenever they have any, are always lateral- 3. The third 

 great division includes all the animals known under the name Zoophytes, 

 to which Cuvier gives thename of Radiata. In the preceding divisions, the 

 organs of movement and sensation are disposed symmetrically on the two 

 sides of a common axis. In the present, they are arranged circularly around 

 a common centre. In this last division, too, the lowest in the scale of ani- 

 mated beings, the nervous and circulating system almost disappears, the 

 respiratory apparatus is almost always on the surface of the body; and in 

 the greater number, the intestinal canal presents th? appearance of a simple 

 sac without an outlet. The last families of this division present the ap- 

 pearance of a homogenous pulp, indistinctly perceived to possess animal; 

 life, from giving indications of motion and sensation. 



CLASS I. MOLLUSCA. 



Jnvertebral, soft, inarticulated animals, furnished with a more or less prominent 

 head at their anterior part. 



The form of the body in the mollusca is extremely various. It is fre- 

 quently oval, more or less elongated, convex above, and flat beneath, as in 

 the genera Doris, Limax, &c. Ills, also, sometimes oval, and equally convex 

 above and below, as in the Sepias ; elongated and cylindrical, as in certain 

 Loligines ; globular, as in the Octopodes. It is often more or less compress- 

 ed on the sides, as in the ScijUcecb. In very many cases, a large portion of 

 the body is rolled up in a spiral form. A considerable number of these 

 animals present a very distinct separation between the head and the rest of 

 the body, as in the Octopodes. This distinction is sometimes much less 

 marked, as in the genus Dons. The distinction of neck, breast, abdomen, 

 and tail is still less obvious; the body forming only a simple mass. It is 

 seldom that the body is furnished with organs of locomotion, properly so 

 called, although cutaneous expansions are sometimes remarked on the sides, 

 which are subservient to this purpose. The nervous system consists of a 

 95 



