Respiration in Invertebrate Animals. 301 



a definite orbit. This fact alone proves the presence of a heart 

 — if the fluid constituted a chylaqueoas system its movements 

 would be oscillatoiy. 



The Entomostracous Crustacea present few diversities as re- 

 gards the number and disposition of the parts dedicated to the 

 office of breathing. In every species the feet are found to be 

 modified parts. The circulating system is crustacean, not insect- 

 like, in type. The dorsal vessel is a simple tube ; it is in no 

 instance moniliform ; it is not multiplied by valves into indepen- 

 dent contractile chambers. The peripheral circulation is lacunar, 

 not capillary*. In the Branchiopodidie the articulations of the 

 feet expand foliaceously. An augmented surface is thus created. 

 It is utilized respiratorily. By the ceaseless action of the legs a 

 cui"rent in the surrounding element is maintained, which is ap- 

 plied to the purposes of respiration. In ever}* order of Ento- 

 mostracous Crustacea the extreme circulation coincides precisely 

 in every particular with that afterwards to be described in the 

 higher Crustacea. Every appendage of the body is subsernent 

 to the function of breathing. 



In the families Xebaliadse and Branchiopodidae, the abdominal 

 appendages of which are foliaceous, currents of blood can be 

 traced by aid of the corpuscles, traversing irregular passages 

 which coincide with great exactness with those so easily seen 

 in the corresponding appendages of the Macrourous Decapods. 



In several genera of the familiar Daphniadse, and L\Ticeidae, 

 the extreme blood-currents in almost every part of the body can 

 be clearly defined by the eye. In every foot, in every foliaceous 

 appendage, in the very bristles, the act of aeration is accom- 

 plished. The blood-corpuscles in all Entomostraca are crusta- 

 cean in type and structui*e. They are small in number rela- 

 tively to the bulk of the blood. 



In Caprella linearis (PI. XVII. fig. 6), a filiform crustacean, 

 common in the Bay of Swansea, two membranous processes {a) 

 depend from the under surface of the abdomen. By Milne- 

 Edwards they are said to be vesicular. They are really flat. 

 A single current of blood courses round the circumference [U b). 

 The centre of the lamina is parenchymatous. These organs 

 exemplify a principle in the organization of the Crustacea. 

 They prove how little is the measure of the respiratory function 

 in the Crustacea, compared with the high nervous development 

 and active muscularity of these animals. 



* The general facts stated in the text may be verified most readily bv 

 the examination of any of the numerous Entomostraca which inhabit our 

 freshwater pools. Microscopic in size, they admit of being easily sub- 

 mitted to inspection. 



