ARTHROPOD A — CRUSTACEA 30 1 



nor sieve being developed. The digestive fluid is poured into 

 the stomach. 



In the absence of large claws most of the food is moved for- 

 ward to the mouth by the long line of paired gnathobases, to 

 w^hich it is brought by the rhythmical movements of the append- 

 ages in swimming. 



The blood circulatory system is simple. The long, tubular 

 heart, traversing the first eleven thoracic segments with a pair 

 of valvular openings in each, forces the blood to the head and 

 carapace, whence it travels, through the blood sinus surrounding 

 the intestine, to the posterior end of the body, and thence to the 

 limbs, a portion of each of w^hich acts to a greater or less degree 

 as a gill. The movements of the limbs insure a constant renewal 

 of water with its free oxygen. The blood then returns to the 

 sinus surrounding the heart. 



The nervous system differs from that of the crayfish in having 

 the ventral nerve cord entirely double and ladder-like. The 

 sense organs are confined to organs of touch (the setae, especially 

 abundant as fringes to the limbs) and of sight. The latter 

 consist of a pair of compound eyes similar to those of the crayfish, 

 but sessile, and a simple, single, median one. 



Males are few, and reproduction is as a rule parthenogenetic, 

 that is, by unfertilized eggs. 



1 . What is the habitat of Apus ? Its geologic range ? 



2. How does it differ in general appearance from Cambarus ? 



3. In what respects is it more primitive than Cambarus, nearer 

 the trilobites ? Note {a) external appendages, {b) muscular 

 system, (c) digestive canal, {d) circulation of blood, {e) nervous 

 system. 



Estheria (Figs. 133, 134). Devonian to present. 



Carapace developed into a horny shell of two thin, rounded, 

 inequilateral valves, united by a straight dorsal margin. The 

 surface of each valve is covered wdth concentric ridges, between 

 which are interlacing striae. The umbonal region sometimes 

 bears a strong muscle node. 



