236 THE VITAL FUNCTIONS. 



globular dilatations (e), followed by a still larger, 

 and more elongated oval receptacle (f), which 

 completes the semicircular sweep taken by the 

 vessel in bending round the sides of the body, in 

 order to join the dorsal vessel (g, g), in which all 

 the other four communicating vessels, presenting 

 similar dilatations, terminate. 



The larger Crustacea possess a circulatory appa- 

 ratus still more extensive and complete, accom- 

 panied by a corresponding increase in the energy 

 of the vital functions. As we follow this system in 

 the more highly organized tribes of this class, we 

 find the powers of the dorsal vessel becoming more 

 and more concentrated in its anterior extremity ; 

 till in the Decapoda, a family which comprehends 

 the Lobster and the Crab, we find this part dilated 

 into an oval or globular organ, with very muscular 

 coats, capable of vigorous contractions, propelling 

 its contents with considerable force into the vessels, 

 and having therefore a full claim to the appellation 

 of heart. The circulation is here conducted nearly 

 on the same general plan as in the Annelida ; the 

 blood from the different parts of the body being 

 collected by the venae cavae, which are very capa- 

 cious, and extend, on each side, along the lower 

 surface of the abdomen. They send out branches 

 to the gills : and these branches, at their origin, 

 suddenly dilate, forming large receptacles, termed 

 simiscs, where the blood is allowed to accumulate, 

 and where, by the muscularity of the expanded 

 coats of the vessels, it receives an additional impe- 

 tus. From the branchiae, the blood is returned by 

 another set of veins to the heart, and is propelled 

 by that organ into the systemic arteries. Fig. 354 



