178 CLASSIFICATION OF THE ANIMAL KINGDOM. 



The animal itself hangs loose in the interior of this outward covering, and 

 possesses a simple heart, a mouth which is peculiarly situated very low 

 down in the body of the animal, a short oesophagus, a simple stomach 

 surrounded by a liver, and a long intestinal tube. The reproductive system 

 is of the simplest form, and the nervous apparatus but slightly developed. 



The most peculiar part of the anatomy of these animals is that connected 

 with their means of respiration. A considerable part of the interior of the 

 body consists of a thin vascular membranous cavity, analogous to the mantle 

 of other Mollusca, and this is covered all over with a perfect net-work of 

 veins, and its interior surface studded with vibratile cilia. Into this cavity 

 the surrounding water is being continually drawn, and, as it courses over 

 the net-work of veins, purifies the blood, which is then conveyed back to 

 the system. This current of water also brings in the materials used for 

 food, which are thus conveyed to the mouth; and in order to prevent the 

 intrusion of any foreign substance which might be deleterious to the functions 

 of the body, a simple remedy is provided, for round the outside of the 

 entrance into the cavity is arranged a row of delicate tentacula, which 

 feel the various atoms brought in by the water, and, if fit for food, they 

 allow them to pass, if not, they reject them as unsuitable. Some of the 

 other families of this group are most extraordinary. Some are not attached 

 like the Aacidians, but swim freely about in the water. Some, as the 

 Salpce, have a long thin riband for a branchial organ, over which the blood- 

 vessels ramify, instead of lining the respiratory cavity, as in the Ascidians. 

 These animals possess also another most striking peculiarity; they are solitary 

 and aggregate in alternate generations; the solitary individuals produce an 

 offspring which lives gregariously, and they again produce solitary individuals. 

 Numbers of them swim about fastened together like a chain. Other families, 

 as the Botryllides, are at first free, like the Salpce, and afterwards become 

 fixed. In our own seas we have specimens of all the different genera, 

 and they are very abundant. 



The next order, the Gonchifem, consists of animals which inhabit bivalve 

 shells. Each valve or side of the shell is lined internally with a delicate 

 membrane, called a mantle, bordered with a fringe of retractile filaments, 

 and the uses of this mantle are very various. It is the sole origin of the 

 formation of the shell, and of the spots or streaks of colour with which 

 many species are covered; it also constructs pearls by its nacreous secretion; 

 and in the Scallop tribe it is studded all round with bright specks, which 

 are supposed to be organs of sight. In some species, instead of simply 

 lining the two valves separately, it is united along the edges, so as more 

 or less to enclose the body of the Mollusk, while in many other species 

 it is not confined to the limits of the shell, but protrudes to a considerable 

 distance in the form of a single or double cylinder, fringed at its extremity. 



