SANGUIFEROUS SYSTEM. 463 



attached, and all these cavities of the heart are inclosed in a 

 distinct pericardium. The great aortal trunk is here given 

 off from that extremity of the ventricle, which the intestine 

 first perforates, and in passing inwards it sends off large 

 arterial branches to the abdominal viscera, to the round 

 muscle of attachment, like that of a spondylus or an ostrea, 

 which fixes the hatyotis to its convex shell, and the vessel 

 then continues forwards to supply the parts of the head, so 

 that this animal presents the nearest approach to the conchi- 

 fera in its mode of circulation. A similar division of the 

 auricle was observed also by Poli in the chiton, where each 

 division appears to communicate by two distinct openings 

 with the ventricle. The passage of the intestine through 

 the heart, thus so frequent in conchifera and gasteropoda, 

 may add by exosmosis further nutriment to the blood, where 

 an absorbent system distinct from the veins is not yet deve- 

 loped, or may assist in excreting some material from the 

 blood into the anal portion of the intestine, before that fluid 

 is distributed for the nourishment of all parts of the body. 

 It is obvious also that the constant contractions of the mus- 

 cular ventricle, compactly embracing the inert terminal part 

 of the intestine generally charged with the residue of diges- 

 tion, must mechanically assist the passage of that matter 

 through the canal. 



As the branchiae in the naked tritonice are disposed along 

 the sides of the upper part of the body, the heart, consisting 

 of a single auricle and ventricle, is situated between these 

 organs in the middle of the back, and the aortal trunk pro- 

 ceeding from the fore part of the ventricle, divides at its 

 origin into three principal branches, the posterior of which 

 is distributed chiefly on the genital organs, the middle branch 

 on the digestive, and the anterior larger branch on the mus- 

 cular and sensitive organs of the anterior and lower parts of 

 the body. In other genera where the branchiae are thus 

 disposed equally on the two sides, as in scyllaa, tethys, and 

 others, the cavities of the heart have the same position as in 

 the tritonia ; but when the gills are confined to the left side 

 of the body, as in the great order of pectinibranchiate gaste- 

 ropods, the heart is found to occupy the same side, as seen 

 in the buccinum undatum, (Fig. 22.) where the arterialised 

 blood from a larger (22. g.) and a smaller (22. i.) pectinated 



