_The Lophobranchii_ include the pipe fish, and other fishes of similar form. _The Plectognathi_ comprise the very singular forms of the balloon-fish, the sun-fish, and other similar fishes. _The Chondropterygii, or Cartilaginous fishes_, are divided into three orders, viz. _the Sturiones_, or sturgeon family; _the Selachi_, or sharks and rays, including the torpedo; and _the Cyclostomi_, or lamprey family. The last two orders were included by Cuvier in a single one. THE MOLLUSCOUS ANIMALS Have no bones except their shells. Their sense of feeling appears to be very acute, but the organs for the other senses are either wanting or very imperfect. The blood is cold and white, and the heart often consists of only one ventricle; a few of them have imperfect lungs, but the greater number breathe through gills. They have all the power of remaining a long time in a state of rest, and their movements are either slow or violently laborious. Some of them appear incapable of locomotion. They produce their young from eggs, but some lay their eggs on a part of their own body, where the young are hatched. The following are Cuvier’s six classes:-- 1. _Cephalopoda, or Head-footed Mollusca._ These animals are furnished with long fleshy arms or feet, proceeding from the head, which is not distinct from the body, and on which they crawl. There is only one order, which includes the cuttle-fish, nautilus, and belemnites. 2. _Pteropoda, or Wing-footed Mollusca._ These animals have two membranous feet or arms, like wings, proceeding from the neck. There is only one order, which contains six genera, the best known of which is the Hyalæa, the shell of which is commonly called Venus’s chariot. 3. _Gasteropoda, or Body-footed Mollusca._ All these animals crawl with the flat part of the body, which acts as a kind of sucker. There are nine orders in Cuvier’s system. The common snail will give an idea of the habits of the class. 4. _Acephala, or Headless Mollusca._ These animals have no apparent head, and breathe by means of branchiæ, which are generally ribbon-shaped. Most of them are enclosed in a bivalve shell, but some are naked; the former are the _Testacea_ of Cuvier, and the _Conchifera_ of Lamarck; the latter are the _Tunicata_ of Lamarck. They form two orders. 5. _Brachiopoda, or Arm-footed Mollusca._ These animals also have a bivalve shell; but they have no true branchiæ, and their respiration is effected by the agency of the mantle. They have two spiral arms.