538 GASTEKOPODA. 



times be observed to stop for a short period. When the animal begins 

 to grow weak, and the circulation becomes enfeebled, it is even possible 

 to follow with the eye any given globule during its passage through the 

 peri-intestinal cavity, and through the heart into the aorta. 



(1423.) In studying the anatomy of Haliotis, Milne-Edwards* ob- 

 served that, although injections thrown into the heart were easily made 

 to fill the general arterial system, so as to exhibit the arteries supplied 

 to the liver, to the stomach, and internal viscera generally, and also to 

 render visible even the capillary vessels, in the head he invariably 

 found the injection extravasated so as to fill a great cavity, in which 

 were lodged the brain, the salivary glands, the pharynx, and all the 

 muscles belonging to the oral apparatus. At first it was supposed that 

 this extensive extravasation was caused by some rupture of the vascular 

 parietes ; but after many unsuccessful attempts it was at last discovered 

 that, on attempting to follow the course of the aorta into the head, it was 

 impossible to find any trace of that vessel beyond the point where this 

 extravasation invariably began to show itself : at this place, indeed, 

 the walls of the great artery entirely disappeared, or, rather, became 

 confounded with the membranous septum that here separates the abdo- 

 men from the cephalic cavity : neither could any continuity be traced 

 between the arterial trunk, after its entrance into this extensive cavity, 

 and the arteries proceeding from it to ramify in the fleshy portion of 

 the foot, although these latter were invariably well filled with the 

 coloured injection employed ; and it soon became evident, from numerous 

 observations, that in this Gasteropod a free communication is normally 

 established between the great arterial trunk of the body and the cephalic 

 cavity, wherein are lodged the principal nervous centres and the whole 

 anterior portion of the digestive apparatus, and that this cavity, in the 

 living animal, is filled with arterial blood. In fact, the aorta having 

 reached the spot where the digestive canal curves downwards to descend 

 from the upper aspect of the pharyngeal bulb into the abdominal cavity, 

 it plunges directly into a wide space or lacuna which surrounds the 

 pharynx and occupies all the front part of the head, taking the place of 

 the cephalic portion of the aorta ; and the arterial blood poured by that 

 vessel into this space directly bathes the brain, the muscles of the pro- 

 boscis, and all the anterior part of the alimentary canal, after which it 

 goes to supply the muscles of the foot and the cephalic appendages. 



(1424.) But there is one circumstance connected with this arrange- 

 ment which appears even still more strange, namely that, while a por- 

 tion of the general cavity of the body thus completes the vascular appa- 

 ratus, the aorta to a certain extent acts as an abdominal cavity ; for in 

 its interior there is lodged a part of the digestive apparatus. 



(1425.) To ascertain this fact it is only necessary to slit open the 



* "Observations sur la Circulation chez les Mollusques," par M, Milne-Edwards 

 (Ann. des Sci. Nat. 1847). 



