258 SUPPLEMENT 



number of vessels, which proceed to unite in a ventral canal, 

 situated underneath the intestine, and this canal distributes it 

 to the entire body, from whence it returns to the heart, through 

 a thick vena cava. 



Thus the circulation is double, as the heart must be con- 

 sidered as the pulmonary ventricle, and the ventral canal as 

 the aortic ventricle. 



In the squillae, the heart is elongated into a thick fibrous 

 vessel, passing along the back and the upper part of the tail. 



That of the small entomostraca, such as daphnis, lynceus, 

 limnadia, is small, globular, situated near the back, above the 

 intestinal canal, and its contractions are very perceptible. 



In the limulae it is a thick vessel, furnished within with 

 fleshy columns, running along the back, and giving out, like 

 that of the squillae, branches from both its sides. 



Finally, in branchipes, we see from the head as far as the 

 last articulation but one of the tail, a brilliant organ, perfectly 

 diaphanous, which is composed of a series of utricles or little 

 cells, corresponding in number to that of the rings of the body 

 (eighteen or nineteen), which contract and dilate successively 

 with considerable quickness, with movements which may be 

 compared to those of the systole and diastole. This organ is 

 very analogous to the dorsal vessel in insects. 



Respiration is a very active function in the Crustacea. The 

 respiratory organs are voluminous, and of two kinds, gills or 

 branchial laminse, and kinds of air sacs. 



The gills are sometimes concealed, sometimes visible ; they 

 are frequently situated on the sides of the body, but often also 

 on the posterior extremity of its inferior face. They are 

 almost always annexed to the base of the ambulatory feet, or 

 to that of the most exterior parts of the mouth ; but also, in 

 many cases, they of themselves alone constitute feet, which 

 serve at the same time for locomotion and for respiration. 



In the brachyurous decapod Crustacea, they are placed at 



