282 



COMPAEATIVE ANATOMY. 



aa 



a(r 



V.l;f 



large ventral siuus, placed at tlie base of the gills (in the so-called 

 sternal canal). Each gill (//r) receives a vessel from this sinus 



(branchial artery). From 

 the gills the blood passes 

 into the branchial veins 

 {vhr), six or seven of which 

 arise on either side of the 

 ]:)ericardial sinus, into -which 

 they often open by funnel- 

 shaped mouths. 



The valves of the venous 

 ostia are to be regarded as 

 special differentiations of 

 the heart, which aid in 

 dividing it into separate 

 chambers when it is elon- 

 gated. 



The circulatory system 

 of the Poecilopoda combines 

 several of these different 

 stages ; their elongated 

 heart lies in a pericardial 

 sinus, from which it re- 

 ceives blood by seven pairs 

 of ostia; it does not onlj 

 give off arterial trunks an- 

 teriorly and posteriorly, but 

 from the sides also, as do 

 the Stomapoda. 



§ 218. 



The circulatory organs 

 of the Tracheata very 

 much resemble the elon- 

 gated multi - camerate 

 hearts found in some Crus- 

 tacea; and the differences 

 are due rather to the de- 

 gree to which the vascular 

 system which passes froin 

 the heart is developed. This 

 again is affected by its 

 relations to the respiratory 

 organs, for when they are 



Fig. 145. Diagram of the circulatory sj-stem 

 of a Lobster, o Eye. oe Lateral antennEfi. 

 ai Mesial antennae. br Brancliise. c Heart. 

 J3C Pericardium. oo Median anterior aorta. 

 aa Hepatic artery, a-p Posterior artery of tbe 

 body. aTrunkof the ventral arteiy. ai' Anterior 

 ventral artery, v Ventral venous sinus, v hr 

 Branchial veins. The arrows indicate the 

 direction of the current of blood. 



limited to a small space the 

 blood-vessels are more perfectly developed, while when the respira- 

 tory organs are distributed over the whole body the arteries are less 

 well developed. The Protracheata appear to resemble them in this. 



