The anatomy of Lottia gigantea Gray. 21 



Thus boundaries of the pericardium are formed in this direction. 

 Posteriorly the auricle extends quite to the aperture of the funnel 

 leading toward the kidney. Along the whole of the right side, fre- 

 quent and regular openings lead into the auricle from spaces in the 

 mantle hood, into which sinus the vessel from the ctenidium opens 

 (Ct. V). Blood thus enters the auricle from 1) the circum-pallial 

 sinus; 2) from the ctenidium; 3) from the pallial hood. By far the 

 greater amount comes from the mantle which is the chief respiratory 

 organ. 



The auricle is a rather thin sac with prominent muscular cords 

 ' running lengthwise and smaller ones crossing at various angles. Its 

 shape changes according to different degrees of contraction. Fig. 24 

 gives an idea of the appearance during systole. 



The ventricle may be said to occupy the posterior portion of the 

 pericardium, along that edge of which, its walls are fastened. In 

 front, it is joined with the auricle by a fairly large opening, guarded 

 on the ventricle side by a two-lipped valve (Val). From the corners 

 of this valve prominent cord-like muscles lead off and attach to the 

 ventricle walls. From the lower side of the ventricle a long slit-like 

 opening with muscular lips, leads directly to the aorta. Two portions 

 are easily distinguishable in the ventricle. The anterior half as shown 

 in Fig. 24 has heavy muscular walls, with cords running in various 

 directions, but mostly lengthwise and circularly. A view of the in- 

 terior presents that peculiar and characteristic fenestrated appearance, 

 seen in so many hearts, both vertebrate and invertebrate. The posterior 

 half has thin walls, translucent and delicate, made up mostly of fibres 

 running at right angles to the prominent cords in the anterior half. 

 A crease leads to either corner of the ventricle from the two ends of 

 the aorta opening, and from this crease free slender bands of muscle 

 lead up to the dorsal wall of the ventricle. These muscle bands seem 

 to act as stays preventing the ventricle from encroaching too much 

 on the aorta and thus stopping a ready flow of blood. 



I have noticed in live animals in captivity that the tip of the 

 ctenidium often vibrates rhythmically, there being usually about forty 

 of these vibrations per minute. Each vibration consists of two parts 

 as though recording the contraction of both auricle and ventricle. A 

 pause is succeeded by a slight moving of the tip of the ctenidium, 

 which is followed almost at once by a strong vibration, then a pause. 

 These animals were in a perfectly normal condition so that if the ex- 



