320 LECTURE XV. 



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 Ci'ustacea.* An aortic trunk proceeds from each extremity 

 of this heart. The anterior 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 supplying 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 intestine, 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 again divides. Throughout all this course 

 the arterial is so closely connected with the nervous system as to be 

 scarcely sepai'able or distinguishable 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 appen- 

 dage. The aei'ated 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 

 originally developed from the flabelliform appendage, although they 

 ultimately 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 Amphipoda ; and to the 

 abdominal limbs, as subdivided lamellas, in the Isopoda : the 

 branchial plates expand into vesicles attached to the thoracic feet 

 in the Lccmodipoda. In the Stomapoda, the respiratory plates are 

 also external, and are appendages of distinct locomotive organs, and 

 each plate is divided into a series of small filaments or tubes ; so as 

 to resemble a broad feather : their position is abdominal. Similarly 

 formed external gills are appended to the thoracic segments in the 

 Thysanopoda. 



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



