There is no division of the canal into small and large intestines, as in the higher classes ; or rather, among the molluscs, the relative size of the different parts is re versed. Here the pyloric extremity is usually the largest, while the anal is more slender. The intestine, like fishes, is short in proportion to the length of the body, and, in its course, is subject to few turns. The anus is, in some, placed on one side of the body ; in others it is terminal, whY.e, in a few, it opens on the back.
The digestive system is thus more simple in its struc ture than in the higher classes. It possesses neither pancreas, spleen, nor mesentery. The calls of hunger are often at distant intervals, and the power of abstinence is great.
The characters furnished by the digestive system are extensively used in the inferior divisions of molluscous animals. The form of the lips, the position of the mouth and anus, and the structure of the stomach, de serve to be attentively considered, as indicating the ha bits of the species.
6. Circulating mechanical process by which the food is converted into chyle has not been satisfactorily traced, nor has the existence of lacteals for the absorption of the chyle been demonstrated. In this class of animals, the veins seem to perform the of fices both of lacteals and lymphatics.
The blood in this tribe of animals is white, or rather of a bluish colour. Its mechanical and chemical con stitution have never been successfully investigated.
In the employment of the words right and left, to express the relative position of the cavities of the heart in the inferior animals, much confusion must necessarily arise; or rather, other terms must be used to render our descriptions intelligible. The nomenclature of that learned anatomist, Dr. Barclay, appears to obviate all difficulty in reference to the ambiguity of words indi cative of position, and to convey in its expressions an idea of the uses of the vessels which are alluded to. Thus the pulmonic vessels include all those which bring the blood collected from different parts of the body to the lungs, such as the yew cava, right auricle, right ventricle, and pulmonary arteries. The systemic vessels are those which convey the blood from the lungs to the different parts or the body, including the pulmonary veins, left auricle, left ventricle, and aorta. it is our in
tention to employ these terms as we proceed.
The circulating system of molluscous animals exhibits very remarkable differences in the different classes. In all of them, however, there is both a systematic and a pulmonic system of vessels, as in the higher classes of They mar be divided, in reference to their circulating system, into four groups. in the first, the blood is collected from the different parts of the body into a vena cava. This vein is divided into two branches, to each of which there is a ventricle attached, and from each ventricle an al wry proceeds to the gills or lungs in its neighbourhood. Each of the two divisions of the lungs gives rise to a vein ; these terminate in a single ventricle, from whence the blood is transmitted to the different parts of the body. There are here one systemic and two pulmonic ventricles. This arrange ment prevails among the cephalopoda.
In the second group, the vessels in which the blood has been collected from the different parts of the body proceed directly to the lungs or gills, without the inter vention either of auricle or ventricle. The systemic veins which have absorbed the blood from the lungs pour it into an auricle, from whence it passes into a ventricle, to be distributed throughout the body. Here, then, there arc neither pulmonic auricles nor vent idles, while there is one systemic auricle and ventricle. This dristribution prevails among the gasteropods, pteropoda, and inequivalve conellifera. In these last, however, as the oyster and scallop, the auricle is bilobate, making an approach to those of the following division.
In the third group, like the second, there are neither pulmonic auricles nor ventricles, the pulmonic veins pro ceeding directly to the lungs or gills. The systemic veins, however, terminate in two auricles, these empty their contents into one ventricle ; so that there arc two systemic auricles and one systemic ventricle. This structure appears in the equivalve conchifera.
In the fourth group there are no pulmonic auricles or ventricles, neither systemic auricle ; but there are two systemic ventricles. This includes the animals of the brachiopoda.