180 LECTURE XV. 



heart by the large orifices on the sides of the ventral aspect {d, d), 

 regurgitation being likewise prevented by two semilunar valves at each 

 orifice. 



The muscular fasciculi of the ventricle are so arranged as to leave 

 tolerably distinct chambers for each series of arterial orifices. The 

 strongest bands or columns are transverse, and dorsad of the branchial 

 apertures : anterior to these is a chamber receiving the carbonized 

 blood from the two anterior dorsal apertures ; it is partially divided, 

 the upper chamber giving off (/) the ophthalmic, and (g) the anten- 

 nal arteries. The lower chamber sends off (^, k) the hepatic arteries. 

 The larger posterior chamber receives the two lateral venous aper- 

 tures, and gives off (h) the superior caudal artery, and (i) the sternal 

 artery, the only one which has a pair of semilunar valves at its origin. 



A portion only of the ordinary venous blood is returned directly 

 by the four valvular orifices just described. The blood is returned 

 from the maxillae and legs to a series of inferior lateral sinuses at the 

 bases of the branchiae ; from these sinuses it passes, by vessels per- 

 forming the office of branchial arteries along the outer margin of the 

 gill, and after being distributed over the branchial lamellae, is collected 

 into the vein along the inner margin of the gill, and by the union of 

 the branchial veins into one trunk, on each side, is poured into the 

 ventricle by the two orifices on its under surface (d, d). 



We may trace in the heart of the Crustacea a gradational series of 

 forms, from the elongated median dorsal vessel to the shorty broad, 

 and compact muscular ventricle in the lobster and the crab. In 

 all the Crustacea, as in all the other articulate animals, the heart 

 is situated immediately beneath the skin of the back, above the 

 intestinal tube, and is retained in situ by lateral pyramidal muscles 

 (Jig. 91, a, a). In the lower, elongated, slender, many-jointed 

 species of the Edriophthalmous Crustacea the heart presents its vasi- 

 form character : it is broadest and most compact in the crab. 



In this series we may trace a general correspondence in the pro- 

 gressive development of the vascular as of the nervous system, con- 

 comitant with the concentration of the external segments, and the 

 progressive compactness in the form of the entire body. But there 

 is a remarkable exception to this concomitant progress in the Limulus, 

 indicative, with the general condition of the instruments of locomo- 

 tion and respiration, of the essentially inferior grade of organisation 

 of that genus, which, as has already been observed, seems to be the 

 last remnant of the once extensive group of Trilobitic Crustacea, 

 which swarmed in the seas of the ancient secondary periods of the 

 earth's history. 



We have seen in the compact and broad existing representative of 



