776 



CRUSTACEA. 



The heart is always found in the median 

 line of the body, and lying over the alimen- 

 tary canal near the dorsal aspect. Its form is 

 various ; in the Decapods it is nearly square, 

 and lies in the middle and superior part of the 

 thorax, being separated from the carapace by 

 tegumentary membranes only, and may be 

 seen in the space included between the two 

 vaults of the flancs. In structure it appears 

 to be composed by the interlacement of nu- 

 merous muscular fibres, fixed by their extremi- 

 ties to neighbouring parts and passing to some 

 distance over the aggregate at either end, 

 so that the whole organ brings to mind such 

 a figure as would be formed by the super- 

 position of a number of stars the rays of 

 which do not correspond. In the other orders 

 this general form of the heart varies conside- 

 rably, from the figure of an oblong square of 

 rather inconsiderable size, as it occurs in the 

 Decapoda (Jig. 418,^,10 that of a long cylin- 

 drical vessel extending through the whole 

 length of the body as it appears in the Stoma- 

 poda (Jig. 419), and the Edriophthalmians. 

 In the former of these it gives origin to six 

 vascular trunks, three of which issue from the 

 anterior edge, and three from the posterior 

 surface ; each of the six openings is closed by 

 a valvular apparatus which prevents the regur- 

 gitation of the blood. 



The first of the three anterior vessels is 

 situated in the median line and is distributed 

 to the eyes, in consequence of which we have 

 entitled it the ophthalmic artery (a, Jig. 418). 

 Lodged within the substance of the general te- 

 gumentary membrane, it continues its course 

 without undergoing any subdivision along the 

 median line through the whole length of the 

 thorax, until, arrived opposite the eyes, it sub- 

 divides and terminates in two branches which 

 penetrate the ocular peduncles. 



On the two sides are the two antennary 

 arteries. They run obliquely towards the an- 

 tennae, sending off numerous branches to the 

 tegumentary membrane in which they are at 

 first lodged; they then plunge more deeply, 

 sending branches to the stomach and its mus- 

 cles and to the organs of generation, between 

 which they insinuate themselves by following 

 the folds of the same membrane which parts 

 them. Lastly, each of these vessels subdivides 

 into two branches, one of which proceeds to the 

 internal and the other to the external antenna. 



Two hepatic arteries arise from the fore part 

 of the inferior surface of the heart, and pene- 

 trate the liver, there to be ramified ; but they 

 are only found double and distinct from one 

 another so long as the liver is met with divided 

 into two lobes, as it is in the River-crab and 

 Lobster. 



From the posterior part of the same surface of 

 the heart there proceeds a large trunk, which, 

 from its importance, might be compared with 

 the aorta. This is unquestionably the vessel 

 which many authors have spoken of as a great 

 vena cava : we have entitled it the sternal artery. 

 It bends forwards, giving origin to two abdo- 

 minal arteries (o,Jig. 418), dips into the sternal 

 canal, distributing branches to the different 



thoracic rings, as also to the five first cephalic 

 rings, which it passes over in its course. Meet- 

 ing with the oesophagus it bifurcates, but still 

 sends branches to the mandibles and the whole 

 of the anterior and inferior parts of the head. 



The bulb presented by the sternal artery at 

 its origin, in the Macroura, is the part which 

 Willis characterized as the auricle of the heart. 

 As concerns the two abdominal arteries, which 

 may be distinguished into superior and inferior, 

 and which arise from the kind of cross which 

 it forms almost immediately after its exit, they 

 are in precise relationship in point of size with 

 the magnitude and importance of the abdo- 

 men itself. In the Brachyura they are mere 

 slender twigs; in the Macroura, on the con- 

 trary, they are capacious stems, and the inferior 

 of the two sends branches to the two posterior 

 pairs of thoracic extremities. 



The disposition of the three first vessels is 

 the same in the Stomapoda as in the preceding 

 species ; but the great vessel which represents 

 the heart being extended through the whole 

 length of the body, supplies immediately other 

 arterial branches in pairs, and in number equal 

 to those of the rings. 



Fig. 419. 



. 



Arterial system of the SijttiUa. 

 b, heart j a, anterior artery. 



Fig. 420. 



Venous system of the Mtija. 



a, venous sinuses ; b, branchiae ; c, vault of the 

 flancs partly taken away ; d, legs. 



