250

CIRCULATION IN INVERTEBRATED CLASSES.

289. Among the Invertebrated classes generally, the condition of the circulating apparatus differs from that which prevails

Fig. 138. THE SAME, IN THE PERFECT ANIMAL.

throughout the Vertebrata, in one remarkable feature;namely, that whereas in the latter the blood moves in every part of its course through a set of closed vessels, it meanders in the former through a set of channels or sinuses excavated in the substance of the tissues, and communicating with the "general cavity of the body" in the midst of which the viscera lie. Generally speaking, it is in the venous system that the greatest deficiency exists; for the heart usually sends forth the blood by definite arterial trunks, which distribute it by its ramifications through the substance of the various parts of the body; and it is in its course from these to the respiratory organs that it is least restrained within definite boundaries. The degree of this imperfection differs considerably in the several groups of Invertebrata; for whilst, in the highest Mollusca and Articulata, the vascular system is almost as complete as in Vertebrated animals, we find it gradually becoming less and less distinct as we descend, so that in the lower forms of both series it presents itself merely as an extension of the general cavity of the body, and is not furnished with any special organ of impulsion.

290. In the greater part of the MOLLUSCA, the circulation

CIRCULATION IN MOLLUSCA.

251

takes place nearly on the same general plan as in Fishes; the heart having two cavities, and the whole of the blood traversing both the respiratory and the systemic vessels, between each time of its leaving the heart and returning to it again. But this heart is systemic, and not pulmonary; for it receives the arterial blood from the gills, and transmits it to the great systemic artery; and after the blood has been rendered venous by its passage through the tissues of the body, it enters the channels which distribute it to the gills, before being again subjected to the action of the heart. The accompanying figure (fig. 139) of the circulation in the Doris (a kind of sea slug)

a

с

d--

f

b

Fig. 139.-CIRCULATING APPARATUS OF DORIS.

will serve to show the general distribution of the vessels in this group. The heart consists of the ventricle a, whence issues the main artery b; and of a single or double auricle c, in which terminate the veins, d, of the branchial apparatus e. The aerated blood which these convey to the heart, is trans mitted by it, through the artery b, to the system at large; and from this it is collected, in the state of venous blood, by the sinuses which terminate in the large trunk ff. By this trunk it

252

CIRCULATION IN GASTEROPODA AND CEPHALOPODA.

is distributed to the gills e; and thence it returns to the heart, after having undergone aeration. Now if a second heart had been placed on the trunk ff, just as it is about to subdivide for the distribution of the blood to the gills, the circulation would have been analogous to that of Birds and Mammals. There is a great variety in the position of the gills in Molluscous animals, and a corresponding variety in the situation of the heart, which is usually placed near them. In the Doris the gills are arranged in a circular manner, round the termina tion of the intestinal canal; but in many Mollusca they form straight rows of fringes on the two sides of the body, In these last, the heart not unfrequently has two auricles; but these are not analogous to the two auricles of Reptiles; for each has the same function with the other-the reception of the blood from the gills of its own side.

291. There is a very interesting variety in the conformation of the heart in the CEPHALOPODA, or Cuttle-fish tribe; which

Fig. 140.-CIRCULATING APPARATUS OF CUTTLE-FISH.

seems to form a connecting link between the plan of the cir culation that prevails among the Mollusca in general, and that

CIRCULATION IN CEPHALOPODA AND CRUSTACEA.

253 which we have seen in the class of FISHES. The auricle and ventricle of the heart are separated from each other; and whilst the latter remains in the position just described, the auricle occupies the place which the whole heart possesses in the class above. The course of the blood in these animals is shown in fig. 140; where c represents the ventricle or systemic heart, from which arises the aorta a, a, as, av, that supplies the body with arterial blood. The venous blood is returned through the great vein vc, covered with a curious spongy mass cs, the use of which is not known; this also receives the blood from the intestinal veins vv; and it divides into two trunks which convey the blood to the gills or branchiæ (br and br), where it undergoes aeration. On each of these trunks is an enlargement, cb, which has the power of contracting and dilating, and thus of assisting the transmission of the blood through the arteries of the gills, ab. The blood is returned to the ventricle by the branchial veins, vb, on each of which there is another dilatation, bu, which might be regarded as analogous to the auricle of the other Mollusca, but that it is not muscular. Thus in the Cuttle-fish, the blood receives an impulse from the systemic heart, by which it is transmitted into the main artery; and when it returns by the systemic veins, it receives another impulse from the branchial hearts, before it passes through the gills;-an arrangement obviously analogous to that which we meet with in the highest Vertebrata.

292. In the Crab and Lobster, and other animals of the class CRUSTACEA, the blood for the most part follows the same

Fig. 141.-CIRCULATING APPARATUS OF LOBSTER.

course as in the Mollusca, excepting that the heart contains but a single cavity. The arrangement of the circulating appa

254

CIRCULATION IN CRUSTACEA.

ratus of a Lobster is seen in fig. 141, in which a is the heart; b and c, the arteries to the eyes and antennæ; d, the hepatic artery; and e and f, the arteries which supply the abdomen and thorax. The blood that has been propelled through these by the action of the heart, finds its way into the great venous sinus g g, which receives the fluid collected from all parts of the body; from this it passes to the gills, h; and thence it is returned to the heart by the branchial veins, &. Another view of a portion of the circulating apparatus is given in fig. 142, which represents a transverse section of it in the region

b ve C f ob

va

Р

st ce

Fig. 142.-BRANCHIAL CIRCULATION OF LOBSTER.

of the heart, with one pair of gills. The heart is seen at c; and from its under side proceeds one of the arterial trunks which convey the blood to the system. Returning thence, the blood enters the venous sinus s, which has an enlargement at the base of each gill; and this seems to act the part of a branchial heart, like the corresponding enlargement on the branchial vessels of the Cuttle-fish. From this cavity, it is carried by the vessel va into the branchia b; and after it has passed through the capillaries of the gill-filaments, it is collected by the vessels ve, which carry it to the branchial veins, vb, and thence to the heart. The general plan of the circulation in this class is shown in fig. 132.

293. In the class of INSECTS we find a still greater incompleteness in the system of vessels for the conveyance of blood. Arterial trunks can only be traced to a short distance from the dorsal vessel, which answers the purpose of a heart; and the nutritive fluid which they convey is delivered into the channels or sinuses that exist among the different organs.