RESPIRATORY CIRCULATION. 



269 



which distribute the blood to the gills ; but these 

 branches, at their origin, suddenly dilate, so as 

 to form large receptacles, which are called 

 simises^ where the blood is allowed to accumu- 

 late, and where, by the muscularity of the ex- 

 panded coats of the vessels, it receives an addi- 

 tional force of propulsion. From the branchiae 

 the blood is returned by another set of veins 

 to the elongated heart formerly described, and 



propelled by that or- 

 gan into the systemic 

 arteries. Fig. ^54 

 shows the relative si- 

 tuation of these ves- 

 sels, when isolated 

 and viewed from be- 

 hind, in the Maja squinado. c, c, are the venae 

 cavae ; e, e, the venous sinuses above-mentioned ; 

 F, F, are the branchial arteries ; g, the gills, or 

 branchiae ; and i, i, the branchial veins termina- 

 ting in the heart l.* 



In the Mollusca, the heart acquires greater 

 size, compared with the other organs, and exerts 

 a proportionally greater influence as the prime 

 mover in the circulation. In the dev elopement 

 of its structure, in the different orders of this 



* A minute account of the organs of circulation in the Crusta- 

 cea is given by Audouin and Milne Edwards, in the Annales des 

 Sciences Naturelles, xi, 283 and 352, from which work the above 

 figure is taken. 



