CLASSIFICATION OF THE ANIMAL KINGDOM. 177 



explained. The arms, which are contracted and folded up when at rest, 

 are protruded when required by a forcible injection of a fluid to their 

 extremities; their uses are various, the principal one being the procurement 

 of food by means of cilia causing currents in the water. They assist also 

 in opening the valves of the shell by their forcible protrusion, and, probably, 

 when extended answer in some measure the purpose of oars. The mouth 

 is a simple prominent orifice, without any dental apparatus. The alimentary 

 canal varies in the different genera. In Lingula, says Professor Jones, 

 "it is a long and convoluted tube, but without a perceptible stomachal 

 dilatation." In Terebratula, however, there is a large oval stomach, into 

 which numerous ducts leading from the liver open by large orifices. 



In their respiratory system the Brachiopods are very peculiar, for instead 

 of possessing proper branchial organs like other mollusks, the mantle is 

 traversed all over with arborescent blood-vessels, which cause the blood to 

 be aerated by exposure; and this is further carried out by the assistance 

 of the cilia before-mentioned placed round the mantle, which by their 

 vibratile action, are continually pouring fresh currents of water over its sur- 

 face. The cilia placed on the arms do not in any manner assist in carrying 

 out the circulatory process, for the stem itself contains no blood-vessels, 

 and the cilia are of a horny texture. The course of circulation has not 

 been fully made out, but the Brachiopods possess a double cavity or heart 

 for passing the purified blood through the body. The nervous system is 

 also little understood; small ganglia have been detected in various parts of 

 the body, with radiating nervous threads. The muscular system, however, 

 is well developed, and is of a peculiar construction, differing materially from 

 all other bivalve mollusks, while their organs of reproduction are also, like 

 the nervous system, but imperfectly known. The ova, when they exist, 

 are invariably found between the layers of the mantle, as "is the case with 

 the Girrhopodsj but how placed there, and from what source, has not 

 hitherto been made out. 



In the next class, the Tunicata, we find animals of a much simpler form 

 and structure than those we have been considering, some of the families of 

 which, says Professor Jones, "seem to constitute a connecting link between 

 the Mollusca and the Bryozoa, which latter in many points of their anatomy 

 they much resemble." The animals composing this class are not confined 

 in shells, but are enclosed, as their name indicates, in strong flexible bags 

 of a leathery or gelatinous nature. In the most highly- organized family, 

 the Ascidians, this tunic or bag is of a soft texture, and is fixed by one 

 end to any marine substance, while at the other appear two projections, 

 one higher than the other, each having a single orifice at the top. Through 

 the highest, the water used for respiration and the materials for food enter, 

 while from the other are cast out the ova and all excrementitious matter. 



