540 GASTEKOPODA. 



jccts from the posterior margin of the pharyngeal mass, is entirely 

 enclosed within it. This organ, indeed, protrudes to a considerable 

 distance into the interior of the arterial tube ; and it is from the portion 

 of the aorta which thus forms a sheath for the lingual apparatus that 

 several arteries take their origin, the branches of which are distributed 

 to the intestine and abdominal parietes, the orifices of which are discover- 

 able when the tongue is withdrawn from its aortic sheath. 



(1426.) The inferior condition of the circulatory system in the 

 Haliotis is, however, not indicated only by the singular arrangements 

 described above. In that portion of the mantle which is adherent to 

 the shell, and which forms a sort of border to the posterior and lateral 

 parts of the body, arterial vessels seem to be altogether wanting, the 

 whole circulation being apparently carried on by vessels which receive 

 venous blood, derived immediately from the abdominal cavity, to which 

 they partially return it, but at the same time convey a portion thereof 

 into the branchio- cardiac vessels in the immediate vicinity of the heart. 

 The fibrous tissue wherein these vessels are enclosed seems but little 

 calculated to perform the functions of an accessory respiratory appa- 

 ratus ; so that it would appear, from this anatomical arrangement, that 

 all the blood in progress towards the heart is not submitted to the in- 

 fluence of the air, and that it is a mixture of venous and arterial blood 

 that is distributed by the heart throughout the arterial system. 



(1427.) Lastly, it may be noticed that in the cephalic region, where 

 the diiferent organs are in immediate contact with the arterial blood, no 

 traces are discernible either of veins or of Iacuna3 serving to return 

 the blood thus effused to the respiratory apparatus, whereas in other 

 parts of the body venous canals are met with, the disposition of which 

 is very remarkable : these all communicate freely with the abdominal 

 cavity, as is the case in other Gasteropod Mollusca ; but in the liver, 

 the generative glands, and more especially in the urinary apparatus, 

 they nevertheless form true vessels, the ramifications of which are ex- 

 tremely numerous. 



(1428.) In Patella, or the Limpet, the size of the cephalic sinus that 

 receives blood from the aorta is even more remarkable than in the 

 Haliotis : in the Patella, indeed, the tongue is not itself lodged in the 

 aorta, as in the former case, but is enclosed in a membranous sheath ; 

 the sheath, however, in its turn becomes part of an arterial chamber, 

 into which the aorta empties itself. The aorta itself gives off very few 

 branches, while from the lingual sheath arise all the principal arteries 

 of the body. 



(1429.) The arterial blood fills not only the sheath of the tongue, but 

 is likewise diffused throughout the whole cephalic cavity, where it 

 bathes the muscles and nerves in the same manner as in Haliotis ; but 

 the extent of the sanguiferous sinus is much more considerable than in 

 that mollusk. If, indeed, the capacity of these sinuses be estimated, 



