8 NATURAL HISTORY OF ARTHROPODS. 



all parts of the body, and is then collected in venous sinuses (spaces between the 

 various organs and muscles, there being no true veins), goes to the gills, and then back 

 to the heart. The blood is either colorless or of a pale yellowish or reddish hue, which 

 is due to the fluid portion and not to the few colorless corpuscles. 



With the Crustacea the appendages near the mouth, as has been already indicated, 

 are usually modified for the capture and the comminution of food. After being torn 

 into sufficiently small bits to obtain a ready entrance to the mouth, the substances 

 eaten pass through a short oesophagus into that portion of the digestive tract called the 

 stomach, which in the higher groups is divided into two portions. In the anterior are 

 found three hardened, bony teeth which, moved by appropriate muscles, meet together 

 and thoroughly grind the food. When in a sufficiently fine condition it passes into the 

 posterior and smaller chamber, the passage of the larger particles being prevented by 

 a strainer of stiff bristles. From the stomach the partially digested food passes into a 

 long and straight intestine terminating at the posterior end of the body. A liver, 

 which is always very large with the Crustacea, pours its secretions into the intestine. 

 This slight description of the digestive tract applies to the higher forms, and between 

 them and the degraded Rhizocephala, in which the digestive tract entirely disappears, 

 almost every gradation can be traced. A few of the Crustacea live upon vegetable 

 food, some are parasitic, and draw their sustenance directly from the fluids of the body 

 of their host, Avhile the great majority of the class are scavengers, living on decaying 

 animal matter. The immense amounts of animal tissues which these Crustacea will 

 devour almost surpasses belief. The flesh of a large fish when placed in the sea will 

 wholly disappear in a few hours, all being eaten by these useful forms. 



In their manner of respiration the Crustacea present a marked difference from the 

 insects, in that moisture is always necessary. In the lower groups the aeration of the 

 blood occurs at the surface of the body, no specialized organs being developed, while in 

 the higher forms gills are present, and there the principal portion of the oxygenation 

 of the blood takes place. These gills, which are always expansions of the integument, 

 are either borne upon the limbs or the walls of the body immediately adjacent to them, 

 or are limbs themselves, modified so as to expose a large surface to the water. In 

 some cases the gills hang freely in the water, but more generally they are enclosed in 

 special respiratory chambers, which in the Isopoda are formed beneath the hinder seg- 

 ments of the body, and are enclosed by a pair of modified legs which shut together 

 like folding-doors. In the Decapoda the gill-chambers are two in number, there being 

 one on either side of the anterior half of the body. If we examine a lobster, crayfish, 

 or crab, we find that the portion of the carapax immediately above the walking legs 

 is not the wall of the body, but that between it and the true envelope there exists a 

 cavity into which the gills project. This cavity is nearly closed, and in it the gills 

 would have but little chance for exercising their functions were it not for an in- 

 teresting contrivance for constantly renewing the water in the chamber. At the 

 anterior end of the chamber there is a thin, leaf-like organ which in life is in constant 

 motion, thus forcing the water forward, while fresh water enters from behind. This 

 organ is really the exopodite of the second maxilla, and has received the name scaph- 

 ognathite, or the bailing jaw. In a land crab from the East Indies (Birgus latro), 

 Dr. Semper found that by a long life upon the land the gills had become much 

 reduced in size through disuse, and to afford a means of respiration there had devel- 

 oped in the upper portion of the branchial cavity numerous ramified tufts w r ell supplied 

 with afferent and efferent blood vessels, and which can be interpreted only as func- 



