Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/675

Rh CRUSTACEA G41 the form of pyramidal bodies, each consisting of a central ascending stem, with numerous horizontal branches or plates folded close together through which the blood circu lates. There are twenty-two such structures in the lobster, eleven on each side of the thorax, attached to the basal joints of the thoracic limbs, 1 each pair of gills&quot; being fur nished with its epipodits, or upper footlet, which serves to keep the gills apart from each other. In their most simple form they consist of a mere sac-like appendage held by a small neck pendant from the coxal joint, and exposed in the water without protection. In the Decapoda they become more complex in structure and more voluminous, and would be extremely liable to injury if not protected by some means. But, as Spence Bate 2 truly observes, being external to it, they could not be covered or protected by their own somite, as, if it had passed over them, the branchial appendages would have become internal, their character and constitution would therefore be changed; they would cease to be ex ternal ; in fact, they would cease to be branchiae These {ippendages, however, exist as branchiae, and are neverthe less securely covered and pro- FlG&amp;gt; 18 ._ He art of Ilomarus vul- tected; not, indeed, by their garis, Edw., laid open. (Copied own somite, but by the great from Owen s Lectures, p. 318.) development of the mandibu- &quot;.the ophthalmic artery; a, a, the an 1 j i tennal arteries; ft, h, the hepatic lar and posterior antennal somites incorporated together, forming the carapace so char acteristic of the typical Crus tacean. The branchial appendages are thus external in relation arteries;, r , openings to dorsal sinuses protected by semilunar valves; ab, ab, large orifices by which the arterial blo* d from the branchiae enters the heart ; v&quot;, v&quot;, orifices by which lateral sinuses conduct venous blood to the heart ; s, sternal artery ; c, supeiior caudal artery; pm, pm, lateral pyramidal muscles which re tain the heart in situ. N.B. Bristles have been passed through of the body of the animal, but the orifices v&quot;, v&quot;, to indicate their covered over and protected posl by the lateral walls of the carapace. To complete this so as effectually to protect these organs without pressing on them or interfering with their functions, a very considerable amount of lateral development has taken place, and a peculiar reflection so as to bring the margin of the cara pace below the branchial appendages, and to protect them from rude contact with the limbs. Externally, the cara pace covers and protects both ths hepatic and branchial organs ; but, internally, a calcareous wall of demarcation exists bstween the two. This wall, which Milne-Edwards terms the cijtodema, is continued into a thin membranous tissue that makes a distinct and well-defined separation be tween the branchial appendages and the internal system, so that the aqueous element so necessary for the aeration of the blood as it passes through the branchiae may have full power to play upon the gills, and yet leaving no passage that would admit it to the internal viscera so as to derange &quot; The number of branchial pyramids,&quot; says Milne-Edwards, &quot; varies greatly, especially in the Macroura ; at the most it is twenty-two, as in Astacus, and nearly allied species ; in other Macroura the number is eighteen, as in Palinurus, Scyllarus, Penceus ; fifteen in Gebia ; twelve in Pandalus ; ten in Callianassa ; eight iPalcemon; seven only in Crangon, Ilippolyte, Sergestes. In the Anomoura the num ber also varies very much. In the Brachyura we can almost always reckon nine branchioe on each side ; two of this number are, however, merely rudimentary. Sometimes one or more of the last, or last but one, are entirely -wanting&quot; (Todd s Encyclop. vol. i. p. 781). 8 Report on the present state of our knowledge of the Crustacea, part i., British Assoc. Reports, Bristol, 1875, p. 49. the general economy of the animal (Spence Bate). The gills are not ciliated, and thus they require that the water within the branchial ca^ ity in which they are placed should be incessantly renewed by other means. In the crabs two passages communicate with the branchial chamber, one for the entrance, the other for the exit of the water necessary to respiration. The efferent orifice always opens on each side in front of the mouth under the posterior maxilliped. The afferent opening varies greatly in position in the different groups. In the Macroura (lobsters), and in some of the Anomoura (her mit-crabs), the margin of the carapace is not accurately fitted to the thorax along its lower lateral border ; the branchial cavity is thus open along the whole extent of its inferior edge, and so the water finds its way readily into the respiratory chamber. In the Brachyura (crabs), the afferent orifice is more circumscribed, but varies in a still greater degree. In nearly all it exists as a cleft of considerable breadth in front of the base of the first pair of am bulatory appendages between the carapace and the thorax. In the Ocypoda, the third and fourth pair of feet are more closely approximated than the rest, and their margins bear a dense border of long silky peculiarly-formed hairs. Between the basal-joints of these feet, Fritz Miiller has discovered a round orifice opening into the branchial cavity, and he finds this to be a true incurrent orifice for the admission of air or water into the branchial chamber. 3 In the genus Ranina, according to Milne-Edwards, the ordinary anterior entrant orifice to the gill-cavity is altogether wanting ; it is placed instead at the origin of the abdomen. In Grapsus, Fritz Miiller 4 has observed that, when under water, the respiratory in-current enters near the front in the usual manner, but when air is breathed, the anterior incurrent orifice being closed, and the hinder border of the carapace elevated, a wide fissure is opened upon each side above the last pair of feet leading directly into the branchial chamber. In Lcucosia the two apertures are close together, the incurrent opening being situated in front of the mouth, and the water pass ing in by a conduit parallel to the excurrent canal. The circula tion of the medium within the respiratory atrium is brought about partly by the movements of the legs to which the branchiee are attached, and partly by the epipodites which ascend between the gills. The main agent, however, is the &quot; scaphognathite,&quot; a fiabelli- form appendage of the second pair of maxillipeds, which, rising and falling continually, occasions a rapid current from behind forwards in the water, filling the branchial chamber. Branchiae such as we have described, enclosed beneath the over arching lateral walls of the carapace, are specially characteristic of the Decapoda (crabs and lobsters). In the Amphipoda the head shield is small, and no longer covers the thoracic somites, as in the Decapoda. The branchiae, however, are still borne on the coxal joint of the thoracic legs, but they de pend unprotected from each limb, and are bathed in the surround- ing medium, which is made to pass rapidly over them by the action of the abdomina I flabellse. In Squilla wr. find the appendages of the first five pairs of abdo&amp;gt; minal somites devoted to the office of aerat ing the blood ; the branchiae, however, are not included in a cavity, but float freely in the water which bathes the entire surface of the animal. In the Isopoda the abdominal appendages are all devoted to respiration, the anterior and outer pair inJdotca (fig. 19) being specially modified into a strong operculum (op}, opening laterally and shutting over the five pairs of delicate branchial appendages within. In Limulus five pairs of thoracic feet are modified into broad lamellae, to the inner and upper surfaces of which the gills are attached, whilst the most robust and anterior Fid. 19. Branchioe of pair is modified into a broad operculum cover- Idotea. ing the succeeding five bran chigerous pairs, and O p, operculum; br, bran- also the reproductive organs (see fig. 12). chia. The numbers in- In .W(Branchiopoda), save the antenna dicate the segments, and oral appendages of the head, all the other somites bear simple lamelliform gill-feet, of which there are, according to Baird, about sixty pairs, affording an excellent illustration of mere vegetative repetition of parts. Although the act of respiration by gills seems a peculiarly aquatic method of aerating the blood, yet in both the 3 Facts and Arguments for Darwin, by Fritz Miiller. Translated by W. S. Dallas (Murray), 1869, p. 34. 4 On. cit.. p. 31. VT. Si