Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/674

Rh (HO CRUSTACEA This organ, so large in the crab, undergoes great modifications in the various orders ; in the Edriophthalmia only three pairs of biliary vessels, analogous to those of insects, remain. No vessels have as yet been detected by which the chyle or nutritious fluid elaborated by the diges tive processes is taken up, as it passes along the intestinal canal, and transferred to the circulatory system ; we can only, therefore, conclude that it is transferred by absorption to the irregular venous receptacles which are in contact with the walls of the intestines. CIRCULATORY SYSTEM. In most of the Crustacea the circulation is of the same simple character as that observed in the aquatic larvae of insects, save that in the Crustacea the blood is conveyed to the gills for the purpose of oxygenation ; but where no special respiratory organs are developed, the fine hairs and filamentous appendages attached to the feet doubtless subserve that office, or in some the entire surface of the body. The heart consists of an elongated contractile dorsal vessel, larger behind than before, connected anteriorly and posteriorly by several branches with the inferior or return ing vessels, which running along the whole body receive the blood from the anterior extremity, and carry it into the posterior extremity of the dorsal vessel. In the male Entoniscus Cancrorum the heart (fig. 14). is situated in the third abdominal segment. In the Cassidina the heart (fig. 15) is likewise short and furnished with two pairs of fissures, and is situated in the last segment of the thorax and the first of the abdomen. Lastly, in the young Anilocra the heart (fig. 16) extends Fig. 14. Fig. 16. FIG. 14. Abdomen of the male of Entonitctts canmrum. h, heart ; I, liver. (Fritz Mttller). 1 &quot; Fio. 15. Heart of a young Cassidina. (Fritz MUller.) FIG. 16. Heart of a young male Anilocra. (Fritz Miiller.) through the whole length of the abdomen, and is furnished with four or five fissures which are not placed in pairs, but alternately to the right and left in successive segments (Fritz Miiller, Fur Darwin, pp. 41-42). In the Decapoda the heart is placed near the dorsal surface of the cephalic shield immediately beneath the integument above the intestinal tube, and is retained in its place by lateral pyramidal muscles. It consists of a single chamber or ventricle, suspended in a large sac, called the pericardium, but wholly distinct from the part so named 1 By the kindness of his friend, Mr Charles Darwin, M.A., F.R.S., &c., &c., the present writer has been permitted to use a large number of the illustrations from the English edition of Fritz Miiller s admir able little book, entitled Facts and Arguments for Darwin, translated from the German by W. S. Dallas, F.L.S., Assistant Secretary Geol. Soc. Lond. From Fritz Mtiller s store of interesting facts and observa tions the writer has also largely drawn, especially in regard to his researches in the larval development of the Crustacea, and he takes occasion at once to acknowledge the same vrith thauks. in man. The structure of the heart is made up of the interlacement of numerous muscualr fibres fixed by their extremities to neighbouring parts, and passing for some dis tance over the aggregate at each end, the whole structure reminding one of a number of stars superposed on each other, the rays of which do not cor respond (Milne - Edwards). The ventricle has three pairs of apertures so closed by valves as to readily allow the entrance of blood from the pericardium, but to hinder its regurgitation. It has three other pairs of openings, each of which is the commencement of an arterial trunk convey ing blood all over the body. These arteries have valves at their origin, and ramify and end ultimately in capillaries, which open into what are called venous sinuses, because they are channels without any definite shape. The venous blood collects in a great sternal sinus, and thence passes up into the gills to be oxygen ated, after which it proceeds to the pericardium to find its way into the ventricle (fig. 17). From the researches of MM. Audouin and Milne- Edwards, 2 it had been consid- FlG - 17. Diagram of Circulation ered a3 conclusively proved of Lobster (flbmanw^m). ,, ^ i /&amp;gt;, (After Allen Thomson.) the Decapod Cms- n 4 eart. The aovtic heait C on8i. t in(f that in of a single ventricular cavity, and situated below the posterior margin of the thoracic shield, gives off six systemic arteries (A, a), which con vey the arterial blood to the various organs of the body and to the liver (I). The venous blood returning thence in the systemic veins (v, r) is collected on the lower surface of tho body into sinuses (V, V), from which the branchial aiteries (B, B) take their origin; the branchial veins (b) return the blood which has passed through the gills to the heart. See also fig. 18. tacean only aerated or arterial blood found its way into the heart, to be distributed by it over the general system. But Professor Owen has shown 3 that in addition to the two great branchial trunks which pour their streams of aerated blood into the heart from the gills, four other valvular ori fices, two connected with the series of caudal, and two with the series of lateral sinuses, communicate with the ventricle, and return a portion of carbonized or venous blood to the heart; the circulation is, therefore, to some extent mixed, and as both venous and arterial blood reach the ventricle, they are propelled thence through the system (see fig. 18). The returning blood is not redistributed through the liver, as in man, i.e., there is no portal circula tion. There are no lymphatic vessels. The blood is a slightly dusky fluid, containing numerous nucleated corpus cles, which change their form with remarkable rapidity. RESPIRATORY ORGANS. As the type upon which the Crustacean class is constructed is specially fitted for aquatic existence, branchial organs or gills in some form are essential for the aeration of the blood. The appendages which fulfil this office are attached either to the thoracic or abdominal members or to both. Where they are most highly developed, as in the crab and lobster, they assume 3 Recherches Anatomiques et Physiologiques sur la Circulation dans les Crustaces, Ann. des Sciences Nat., t. ii. 3 Lectures on Camp. Anatomy and Physiology of the Invertebrata, 2d edition, 1855, p. 318.