392 



GASTEROPODA. 



across the other riscera, and communicating 

 with the ventricle near its middle. The blood 

 arrives at the heart through four vessels from 

 the long fringe of branchiae, two coming from 

 the anterior and two from the posterior parts. 

 We have already described the disposition of 

 the branchiae in the Tectibranchiate order, but 

 in following the course of the circulating fluid, 

 we shall find in some of the individuals in- 

 cluded in this division circumstances requiring 

 special notice, as being of extreme interest to 

 th physiologist. In Aplysia, the blood re- 

 tuiried from the system is brought by two large 

 ve~>HS trunks to the vena cava or pulmonary 

 artery (fig. 191, % for in this case the same 



vessel performs the functions of both ; these 

 large veins turn round in the vicinity of the 

 operculum, and unite into one trunk prior to 

 their dispersion over the branchial plates, but 

 on opening them at this point so as to display 

 their interior, a most singular arrangement is 

 brought to light; the sides of the veins are found 

 to be formed of muscular bands (c) crossing each 

 other in various directions, and leaving spaces 

 between them ; these intervals are seen even 

 by the naked eye to be apertures establishing a 

 free communication between the interior of the 

 vein and the abdominal cavity, and allowing 

 injection to pass with facility from the vein 

 into the visceral cavity, or from the abdomen 

 into the vein : the anterior portion of each of 

 these vessels may indeed be said to be literally 

 confounded with the general cavity of the 

 the body, a few muscular bands, forming no 

 obstacle to a perfect communication, being the 

 only separation between the two. It is there- 

 fore evident that the fluids contained in the 

 abdominal cavity may in this manner have 

 free access to the mass of the blood as it 

 approaches the respiratory organ, and that the 

 veins can thus perform the office of the ab- 

 sorbent system ; but in what mariner the blood 



is prevented from escaping through the same 

 channels is not at all obvious, although pro- 

 bably during life the contraction of the fasci- 

 culi which bound these apertures may in some 

 measure obstruct the intercourse. It is from 

 this circumstance, and the analogous commu- 

 nication which exists in the Cephalopoda by 

 the intervention of the spongy appendages to 

 the venae cavse found in those Mollusks, that 

 Cuvier was led to the conclusion that in all the 

 class the veins are the immediate agents of 

 absorption, and that an absorbent system does 

 not exist in any but the vertebrate division of 

 the animal kingdom. We meet, moreover, 

 in Aplysia with another peculiarity in the cir- 

 culating vessels; the aorta, shortly after its 

 commencement, divides into two large arteries 

 (h h'\ one of which presents nothing peculiar 

 in its distribution ; but to the larger of the two, 

 whilst still enclosed in the pericardium, we 

 find appended a remarkable structure, the use 

 of which has been hitherto perfectly inexpli- 

 cable : projecting from the opposite sides of 

 the vessel are two vascular crests, represented 

 in i, formed of a plexus of vessels issuing 

 from the aorta itself, and ramifying in an ex- 

 ceedingly beautiful manner through the sub- 

 stance of these extraordinary organs; in other 

 respects the arteries are distributed in the usual 

 manner. The Cyclobranchiate and Scutibran- 

 chiate Gasteropods approximate the testaceous 

 class in many points of their organization, but 

 in none more so than in the position which the 

 heart is found to occupy, and the arrangement 

 of its cavities. In Patella, indeed, the heart 

 is placed in the anterior part of the body, and 

 still conforms in its general structure to the 

 description which we have given above ; but 

 in Oscabrio the auricle is divided into two 

 distinct portions, one receiving the blood from 

 each range of branchial plates; and in Haliotis, 

 Fissurella, Emargenula, and Parmophorus, not 

 only is this division of the auricle met with, 

 but the ventricle, as in many of the testaceous 

 Mollusks, is perforated by the rectum, and 

 the similarity of arrangement which is here 

 presented with what is met with in the Con- 

 chifera will be readily appreciated by a refer- 

 ence to the article which treats of the anatomy 

 of that division of the Mollusca. 



Nervous system. The nervous system of 

 the Gasteropoda furnishes us with the most 

 perfect form of the heterogangliate, or as it has 

 been less happily denominated, cyclo-gangliated 

 type. It consists of a variable number of 

 ganglia or nervous centres disposed in different 

 parts of the body, but connected with each 

 other by cords of communication, and from 

 these ganglia the nerves appropriated to dif- 

 ferent parts proceed. Each ganglion,, therefore, 

 is a distinct brain; and were the preponderance 

 in size to be regarded as the criterion of rela- 

 tive importance, it would not unfrequently be 

 hard to say to which the pre-eminence is due. 

 There is, however, as we shall soon perceive, 

 an uniformity in the arrangement of certain 

 masses, and a regularity in the appropriation 

 of the nerves proceeding from them to parti- 

 cular organs, which leave us little room for 



