8 M. Z. Gerbe on the Vascular and Nervous Apparatus 



sac, varying in form according to the species, with internal mus- 

 cular columns and delicate transparent walls, formed of longi- 

 tudinal and annular muscular fibres intercrossed in various 

 directions ; it presents on each side a single semilunar fissure, 

 to which a valve of the same form is adapted internally. From 

 this contractile sac issue all the arteries which perform the dis- 

 tribution of the blood. 



The second sac, which is much larger and has its walls 

 thinner and less muscular, completely envelopes tht arterial 

 heai;t, and communicates by two or three oblong apertures with 

 the same number of large venous lacunae, which convey the 

 blood to the heart. This enveloping portion of the central circu- 

 latory organ has been assimilated to the pericardium of the red- 

 blooded animals. In this assimilation there is an appearance of 

 truth, if we consider only the form ; but it is far from being 

 exact if we take into account the function, which is very diff'erent 

 in importance from the form. The pericardium in the Verte- 

 brata is an organ closed everywhere, without any communica- 

 tion either with the cavities of the heart or with the vessels 

 which run to it ; here, on the contrary, the sac which has been 

 assimilated to the pericardium directly receives all the blood 

 into its cavity and transfers it to the ventricle. It is inter- 

 mediate between the venous lacunae and the arterial heart, and 

 fulfils exactly the part which, under another form, the auricle, 

 in fishes for example, performs with relation to the vence cavcB 

 and the ventricle. By its functions, therefore, this second 

 cavity would be the analogue of the auricular portion of the 

 heart in Vertebrata. 



Five arterial branches issue from the anterior extremity or 

 half of the central contractile sac; only one springs from its 

 posterior extremity. Of the five anterior arteries, one (the 

 ophthalmic artery) follows the median line, passes directly to 

 the brain, and is distributed in the ocular peduncles. In those 

 species in which the rostrum in the young state acquires the 

 form of a long spine, the ophthalmic artery is produced to the 

 extremity of this appendage, after having furnished a branch 

 to each eye. This arterial branch, which is one of the largest, 

 is furnished at its issue from the heart with a double valve, or 

 rather with two opposite flaps, separated at the base, in contact 

 at the apex, which alternately open and close to let pass the 

 globules of the blood and prevent their flowing back into the 

 heart. The action of these flaps, which is completely inde- 

 pendent of the contractions of the central organ, is sometimes 

 slow, sometimes rapid ; frequently it is even suddenly and 

 momentarily suspended. Two other branches, one on each 

 side, originating a little behind the preceding, also run forward. 



