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 Motiusca. 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 Articutata. 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 ZooruyTEs, 
to which Cuvier gives the name of Raprata. 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 ami- 
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 the 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. 
Invertebral, soft, inarticulated animals, furnished with a more or less prominent 
head at their anterior part. 
Tue 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, Limaz, &c. Itis, also, sometimes oval, and equally convex 
above and below, as in the Sepie; 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 Scyllee. In very many cases, a large portion of 
the body is rolled up ina 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 Doris. 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 
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