MOLLUSC A. 



603 



The stomach, in many rases, is membranaceous, and 

 can scarcely be distinguished from the remaining por- 

 tion of the intestinal canal. In some cases, however, 

 it is strong and muscular like the gizzard of a bird, and 

 even fortified with corneous knobs for the reduction of 

 hard substances. In some species the stomach opens la- 

 terally into the pylorus, and, in a few instances, pos- 

 sesses a spiral cecum attached to it. 



The Itifr in the mollusca is usually of large dimen- 

 sions, and seated close to the stomach, which it in many 

 cases envelopes. It is divided into numerous lobes, and 

 receives numerous blood vessels. There is, however, 

 nothing analogous to the vena porlarum of quadrupeds. 

 The lute is poured in some into the stomach, and 

 in others into the pyloric extremity of the intestine 

 by different openings. There is no gall-bladder. 



There is no division of the c.inal into small and large 

 intestines, as in the higher classes ; or rather, among the 

 mollusca, the relative size of the different parts is re 

 versed. Here the pylnric 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, while, in a few, it opens on the back. 



The digestive system is thus more simple in it struc- 

 ture than in the higher classes. It poses<es neither 

 pancreas, spleen, nor mesentery. The calU of hunger 

 are often at distant intervals, and the power of abstin- 

 ence it great. 



The characters furnished by the digestive system are 

 extensively used in the inferior divisions of molluscous 

 animal*. 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- 

 biU of the species. 



6. Circulating \y>/<rm The mechanical process by 

 which the fool is converted into chyle has not been 

 satisfactorily traced, nor has the existence of iacteals for 

 the absorption of the chyle been demonstrated. In 

 this class of animals, the veins seem to perform the of- 

 fices both of Iacteals and lymphatic*. 



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 animal*, much confusion must necessari- 

 ly 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 ita expressions an 

 idea of the use* of the vessels which are alluded to. 

 Thus the jiulmonic vessels include all those which bring 

 the blood collected from different parts of the body to 

 the lungs, such aa the vena cava, right auricle, right 

 ventricle, and pulmonary arteries. The tyilemic vessels 

 re those which convey the blood from the lung* to the 

 different parts of the body, including the pulmonary 

 veins, left auricle, left ventricle, and aorta. It is our 

 intention to employ thete 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 systemic and a 

 pulmonic system of vessels, as in the higher classes of 

 animals. They may be divided, in reference to their 

 circulating system, into four groups. In the first, the 

 Wood is collected from the different parts of the body 



into a vena cava. This vein is divided into two branches, Molluiet. 

 to each of which there is a ventricle attached, and from ^Y"" 

 each ventricle an artery 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 sin- 

 gle 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 cephal >jnda. 



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 are neither pulmonic auricles nor ventricles, 

 while there is one systemic auricle and ventricle. This 

 distribution prevails among the gasteropoda, pteropoda, 

 and inequivalve conchifera. 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 auricle's 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 are two 

 systemic auricles and one systemic ventricle. This 

 structure appears in the equivalve conchifera. 



In the fourth group there arc no pulmonic auricles or 

 ventricles, neither systemic auricle ; but there are two 

 systemic ventricles. This includes the animals of the 

 brachiopoda. 



In the last group no pulmonic auricle or ventricle, 

 neither systemic auricle can be perceived ; but there is 

 a single systemic ventricle. Animals of the class luni- 

 cala exhibit this simple arrangement. 



The circulating system furnishes few characters which 

 can be employed in systematical arrangements. The 

 structure of the systemic and pulmonary vessels does 

 not appear to be co-ordinate with any particular plan 

 of external configuration and manner, as we see in the 

 case ot the pteropoda and gasteropoda. In these the or- 

 gans of circulation are very much alike, while the exter- 

 nal forms exhibit very obvious differences. 



7. lie<piralnry Syitrm. It is probable that all animals, Hpirto- 

 bowever imperfect, stand in need ofa supply of oxygen. r * '." 

 In proportion a* the circulating system becomes com- 

 plex, the greater is the quantity of oxygen which is re- 

 quired. In the more perfect animals it has been ascer- 

 tained that this oxygen unites with carbon thrown out 

 of the system, and by converting it into carbonic acid, 

 enables it more readily to escape in the gaseous form. 

 In some animals this production of carbonic acid takes 

 place generally throughout the surface of the skin, 

 while in others, particular organs are appropriated to its 

 formation. This is more particularly the case in those 

 animals which, like the mollusca, have pulmonic and 

 systemic blood vessels. When the animal separates 

 the oxygen from unmixed atmospheric air, the organ in 

 which the process is performed is termed lung. When 

 the separation takes place in atmospheric air united 

 with water, the organ is termed a gill. 



The molluscous animals which respire by means of 

 lungs are few in number, and form a very natural tribe, 

 which Cuvier has termed gaiteropodet pulmonet. In 

 them the respiratory organ is simple, consisting of a 

 single cavity, on the walls of which the extremities of 

 the pulmonary artery are spread. This cavity is placed 



