1 10 CLASS CRUSTACEA. 



FIRST ORDER OF CRUSTACEA. 



DECAPODA, 



Have the head intimately united to the thorax, and covered 

 with it by a test or carapace, entirely continuous, but most 

 fi'equently presenting some deep lines dividing it into divers 

 regions, which indicate the places occupied by the principal 

 interior organs. Their mode of circulation offers some cha- 

 racters which distinguish them from the other Crustacea. The 

 heart, very much circumscribed, of a figure approaching oval, 

 and with muscular parietes, gives birth to six vascular trunks, 

 three of which are anterior, two inferior, and the sixth pos- 

 terior. Of the three anterior arteries, the median {the oph- 

 thalmic) is distributed almost exclusively to the eyes ; the 

 two others (the antennary) spread themselves over the 

 carapace, the muscles of the stomach, a portion of the viscera, 

 and over the antenna3 ; the two lower, {the hepatic) carry the 

 blood to the liver ; the last, (or the sternal artery) the most 

 voluminous of all, and which sometimes originates on the left, 

 sometimes on the right of the posterior part of the body, is 

 principally destined to carry the nutritious fluid to the abdo- 

 men, and to the organs of locomotion. It furnishes a great 

 number of vessels of considerable volume, among which we 

 must particularly remark that which MM. Audouin, and 

 Milne-Edwards, name the upper abdominal artery, because it 

 issues from the posterior part of this artery, (a little before 

 the articulation of the thorax and abdomen, vulgarly called 

 the tail) and because it penetrates into the abdomen, (the 

 tail) where it divides itself into two thick branches, continu- 

 ing its passage backward, becoming more and more slender 



