LOGY.
The MOLLUSCA are more complicated with respect to their internal structure than the two preceding classes ; but as their organs of motion are less perfect, they are placed below the crustacea and insects in the general arrangement.
It is difficult to give a general account of the organi zation of these animals. In order to afford a correct idea of their structure, we must explain their classifica tion, and point out the general differences in the orga nization of each order.
According to the latest improvements of Cuvier's sys tem, as explained by Dumeril, the class of mollusca is subdivided into two sections ; one of which contains those mollusca that have a distinct head bearing the principal organs of the senses ; and the other, those that are either without a head, or whose head is confounded with the rest of the body. The former we may call Ce phala, the latter 4celzhala. The former of these sections is subdivided into three orders ;—Cefthalopoda, having about the head tentacula that are very long, and serve instead of feet ; Pteropoda ; that have either no tenta cula ; or such as are very short, but have the body fur nished with membranes that serve as branches or arms ; and Gasteropoda, that have either no tentacula, or such as are very short, and change their place by crawling on the belly. The ?icephala are divided into two orders,
one furnished with Mix, called by Dumeril Brachiopoda ; and those which have no tentacula, to which he has con fined the name of accphala, as these are the only mollusca that entirely want the head.
It is necessary to remark one general division of mol lusca which is often employed by naturalists, and will be referred to in our comparative view of the organs. This is the distribution of those animals into such as are na ked, or have no hard covering ; and such as are covered with a hard shell, and arc called testacea. The mollus ca, with the two following classes, constitute the class of vermes, according to the Linnxan system. See Mot