The anatomy of Lottia gigantea Gray. 23 



enter the visceral artery (Fig. 3 Rt'). At the point where the radula 

 tube pierces the wall of the cephalic artery there is formed an in- 

 conspicuous crescent-shaped opening," conforming to the side of the 

 radula tube. The flap of this opening is so arranged that blood from 

 the heart can pass out into the head cavity or buccal sinus, but the 

 blood in nthe buccal sinus even omitting the greater pressure in the 

 aorta, is unable to enter the artery. 



Before considering the pedal artery and further circulation of the 

 foot it would be well to devote a few words to the buccal sinus. By 

 this is meant, of course, the space about the pharynx, buccal mass, 

 and cerebral, pleural, and intestinal ganglia, with their connectives. 

 As is well known this is a great blood sinus, a distributing center 

 for all the circulation of the viscera (less gonad) and a large part of 

 the foot and body wall. In Fig. 3 the reader will gain an idea of 

 the sinus in Lottia gigantea, remembering of course that all the or- 

 gans are more compactly fitted together, and that only when the 

 sinus is gorged with blood are there such generous spaces between 

 the buccal mass and pharynx. It is evident that all the structures 

 within the sinus are bathed with blood, directly from the heart, mixed 

 with that which has circulated to some extent in the buccal mass. 

 The posterior wall of the sinus is definite and both dorsally and 

 ventrally extends farther backward than in the intermediate region. 

 In other words the viscera encroach on the head cavity bulging the 

 septum out. But in the region of the intestinal ganglia above, and 

 about the pedal ganglia below, there are posterior prolongations of 

 this head cavity. Above, it does not extend farther back than the 

 visceral ganglion as shown in Fig. 3. The cerebro-pleural and cerebro- 

 pedal connectives lie in small side chambers of the main sinus, which 

 are separated from the central cavity by dorso-ventral sheets of muscle 

 and connective tissue (Figs. 1 and 3). These side cavities communi- 

 cate freely with the main head cavity by openings at the front and 

 rear, and along the sides. From the posterior part of the floor of 

 each of these cavities a small artery or sinus leads down into the 

 front portion of the foot (Figs. 24, 27 Ant. P. A) where it divides, 

 joining branches of that of the other side, and sending twigs forward 

 to the very margin of the foot. 



From the buccal blood sinus the following vessels arise : the 

 visceral artery; the two important neural arteries to the foot; two 

 small arteries to the anterior part of foot (anterior pedal arteries al- 

 ready spoken of) ; two or more small arteries to lips, and to tentacles ; 



