In the leech-tribe the vascular system, on the contrary, is more complicated, for the san guiferous circle is composed of four longitu dinal trunks, and the branches which bring them into communication with each other. Of the four longitudinal vessels two occupy the dorsal and ventral aspects of the mesial line, and two the sides of the body. The dorsal and ventral trunks communicate toge ther by dorso-abdominal branches correspond ing to each segment of the body. The lateral trunks also render to the dorsal trunk a series of dorso-lateral branches, and, moreover, mu tually communicate by a series of abdommo lateral branches which glide transversely be neath the nervous chords. The dorsal and ventral vessels are evidently analogous to those which we have designated by the same names in the earthworm and naffs; and the lateral vessels may be compared to the sub-nerval trunk of the earthworm, except that, instead of being single and situated in the mesial line, they form a circle in which the blood undu lates sometimes in one direction, sometimes in another, but always pursuing an opposite course in the two canals. Lastly, in addition to the above general circulation,' there is observed in the leech-tribe something ana logous to the ' lesser circulation,' (fig. 72) : this is effected in the branches (b, e) of the dorso lateral vessels (a), which are for the purpose of bringing the blood into contact with the aerated water contained in the small membranous vesicles (f) situated at the sides of each seg ment of the animal, and opening externally upon the inferior aspect.* Respiration.—From what has been said of the mechanism of the circulation in the annelida, it will be seen that respiration must be effected either in the vesicles above mentioned, or on the surface of the body. Such in fact is the case; the skin is in general the seat of that function ; but in the greater number of instances, the integument, instead of maintaining the same texture throughout, and acting upon the air in the same manner at every point of its extent, presents at particular spots peculiar modi fications, and thus gives rise to special organs of respiration called ' branchim.' The branchiw of the annelida are almost universally membranous appendages, highly vascular, fixed to a certain number of the feet of the animal, or inserted upon the back near the base of these organs.
In the nereida and some other congeneric annelida, the appendages which are designated branchim, and which in fact seem to be in an especial manner subservient to respiration, are simply a kind of papillae or laminated cutane ous productions very little or not at all sub divided, attached either to the extremity or base of the feet and distributed in an almost uniform manner over the entire length of the body, (fig. 64, fa', f.) In the eunice, and other allied genera, their position is the same, but they assume the form of an elongated filament, furnished with a series of prolongations of a similar filiform shape, disposed like the teeth of a comb, and traversed longitudinally by a canal filled with red blood, (fig. 73,f) In the amphinomian family, as in the former groups, these branchim are placed on almost every segment of the body, so that these organs form along the whole extent of the back a double row ; but here their struc ture is more complicated,for the filaments are extremely subdivided, (fig. 63, f.) In the arenicolce, the form of the branchim is almost the same as in the amphinomes, but they are limited in their position to the middle seg ments of the body. In the genus terebella the branchim are also highly ramified vascular appendages to the integument, but their num ber is inconsiderable, and they are all inserted near the cephalic extremity of the back. In
the serpulw, the membrane which forms a sort of thoracic disc near the cephalic ex tremity of the body, ought to be regarded as an organ of respiration, and it is probable that the tentacles surrounding the mouth like a crown of plumes are subservient to the same function.* In the hirudinm respiration is in part effected by the external skin, but there exists in these annelida a series of small mem branous sacs, which communicate externally each by a minute orifice situated on the ven tral aspect of the body : these sacs derive from the numerous vessels which ramify upon their parietes a considerable quantity of blood. Water penetrates into these organs and seems to subserve a true respiratory purpose. These sacs are commonly denominated pulmonary sacs,' and some authors think that they receive into their interior atmospheric air in a gaseous, form. Their number varies from fifteen to twenty, and it may be observed, when a living leech is irritated after being recently removed from water, that a small quantity of liquid escapes from their apertures.
In the lumbrici terrestres there is in like manner found in each segment and on eitherside of the digestive tube, an enteroid vessel folded upon itself, containing a liquid and opening outwardly by a particular pore. These sacs are less vascular than in the leeches ; never theless there is reason to believe that they fulfil an analogous office, and perform a more or less important part in respiration. Lastly, it has been proved that in the annelida there are other pores, placed on the back, which tra verse directly the dermo-muscular envelope, and communicate with a cavity intermediate to the muscles and intestines, and imperfectly divided by transverse septa, into which air or water can penetrate. This structure may, indeed, belong the respiratory apparatus, n but science does not yet possess sufficient data to solve that question. An analogous dis position has been observed in the nals.* Generation.— The generative apparatus is only very imperfectly understood in the anne lida. It appears that all these animals are hermaphrodite, but that they cannot fecundate themselves ; the intercourse of two individuals being necessary for the accomplishment of the act of generation. It is in the earthworm and leech that this part of their anatomy and phy siology has been most completely studied.
In the leeches the sexual apertures are placed at the inferior surface of the body towards the anterior third, and separated from one another by the intervention of five segments. The anterior aperture belongs to the male organs, and at the season of reproduction a filiform and highly contractile penis is observed to be protruded from that part, (fig. 74, 75, a.) This communicates in ternally with a narrow cylindrical canal (b), which in its turn opens into a kind of whitish vesicle of a pyriform shape (c) commonly cal led the vesicula semi nalis. On each side of this vesicle there is an oval whitish body (d) composed of con torted tubes filled with a whitish liquid : each of these organs is a testicle; and they seve rally give origin to a slender vas deferens (fig. 75, e) of the same colour, which opens into the vesicula seminalis. Lastly, from the posterior extremity of the testicle, another fili form duct (f) is continued,which passes back wards on each side of the nervous cord, and gives origin to a series of pedunculated vesicles filled with a whitish fluid similar to that which is contained in the rest of the apparatus. These organs (fig. 74, g) are generally regarded as accessory vesicles, and they vary both in number and form in different species.