CRUSTACEA. 181 



those extinct gigantic Entomostracans, that the nervous system ex- 

 hibits a concentration of its principal central mass around the mouth, 

 analogous to its condition in the common crab, but with a gan- 

 glionic double cord continuing from it. The heart, however, is 

 far from presenting a corresponding degree of concentration : it 

 remains an elongated, fusiform tube, extending parallel with the 

 intestine from the pylorus to the rectum : it is contained in a peri- 

 cardium with thin membranous walls, formed by the central sinus 

 of the venous system, and it receives the blood from that sinus 

 and from the branchial veins by a series of from seven to ten lateral 

 vertical slits, defended by valves as in the higher Crustacea. * An 

 aortic trunk proceeds from each extremity of this heart. The an- 

 terior aorta is the largest, and immediately divides into three branches. 

 The middle and smallest branch passes forwards to the anterior edge 

 of the cephalic shield, following the curve of its middle line, and sup- 

 plying the small median ocelli in its course. The two larger lateral 

 branches form arches which curve down the side of the stomach and 

 the oesophagus, giving branches to both those parts and to the intes- 

 tine, and becoming intimately united with the neurilemma of the 

 oesophageal nervous collar. They unite at the posterior part of that 

 collar, and form a single vessel, which accompanies the abdominal 

 nervous ganglionic chord to its posterior bifurcation where the vessel 

 agaiu divides. Throughout all this course the arterial is so closely 

 connected with the nervous system as to be scarcely separable or dis- 

 tinguishable from it. The branches of the arterial and nervous trunks 

 which accompany each other may be defined and studied apart. 



The posterior aorta is chiefly destined for the supply of the sword- 

 like tail of the Limulus : the first part of its course is wavy, to adapt 

 it to the strong inflections of that appendage. The aerated is mixed 

 with the venous blood in the heart, and is propelled in that mixed 

 condition throughout the body in the Limulus, as in the lobster. 



The respiratory organs in the Crustacea are essentially appendages 

 to the basal articulations of a certain number of the feet, and are ori- 

 ginally developed from the flabelliform appendage, although they ulti- 

 mately become entirely distinct from the extremities in the higher 

 Crustacea. 



The Branchiopods are so called because their fins or feet present 

 the form of simple plates or flattened vesicles, which float in the 

 surrounding fluid, and expose the blood to the oxygen which the 

 water contains. The branchia are appended to the thoracic limbs, 

 in the form of membranous plates, in the Amphjpoda ; and to the ab- 



* Van der Hoeven, loc. cit. pi. 2. fig. 9. 



N 3 



