DECAPODA. 15 



side and above the feet, and formed of venous sacs united 

 in a longitudinal series, or like a chain. It is thrown into an 

 external vessel — efferent — of the branchiae, where it is re- 

 newed and becomes arterial ; thence proceeds into an internal 

 vessel — afferent \ and finally seeks the heart through canals — 

 branchio- cardiac — laid beneath the arch of the flanks. All 

 the canals of a side unite in one large trunk, and open into the 

 lateral and corresponding part of the heart by a single orifice, 

 the folds of which form a double valve that opens to allow the 

 transit of the blood from the branchiae to this visciis, but pre- 

 vents a retrograde motion by closing. Examined internally, 

 the heart exhibits numerous fasciculi and muscular fibres, va- 

 riously intercrossed and forming several small chambers before 

 the orifices of the arteries. These chambers are so many 

 small auricles, which communicate freely with each other 

 when it dilates, but appear to form a similar number of little 

 cells for each vessel when it contracts, their capacity being 

 proportioned to the quantity of blood in their peculiar vessels. 

 These vessels debouche in the interior of the heart by eight 

 openings, the two lateral valvular ones above mentioned in- 

 cluded. Such, with the exception of some modifications(l), 

 is the general system of the circulation in the Decapoda. 



The superior face of the brain(2) is divided into four lobes, 

 each of the two middle ones furnishing from its anterior mar- 

 gin an optic nerve that plunges directly into the pedicle of 

 the eye and there divides into numerous filaments, each of 

 which is destined to a facet in the cornea of that organ. 



hearts, replace the right ventricle. The focus of the circulation, highly concen- 

 trated in the first of the Vertebrata, thus becomes gradually weaker, so that finally 

 there is no circulation whatever. The dorsal vessel of Insects would then be the 

 mere rudiment of the heart of the MoUusca and Crustacea." I will add, that 

 twenty-five years ago, in my Hist. Nat. des Crust, et des Insedes, I rectified the 

 error of Roesel respecting the nervous cord of the spinal marrow, which had been 

 taken for a vessel. 



(1) See general observations on the family of the Macroura. 



(2) These observations are extracted from the Legons d'Anatomie Compar^e of 

 Ilaron Cuvier. For other details and particular facts see the Memoir of Messrs 

 Audouin and M. Edwards, loc. cit. 



