The anatomy of Lucai)iua creuulata. 461 



in a fixed specimen that they closely resemble nerve cords, and one 

 might be led to think that they were nerves corresponding to the sub- 

 radular of the chitons. In a specimen of this size, however, it is easy 

 to see the branching muscular fibres at either end. The muscular 

 structure of the mantle and foot I have not taken up since they are 

 very similar to others that have been described. 



Vascular system. The Fissurellidae have two well defined 

 auricles with a ventricle situated between them (Fig. M). The auricles 

 have thin, fibrous walls. They are attached along their outer front 

 edges to the anterior wall of the pericardial cavity. A row of glandular 

 appendages fringes each at the back (Fig. 9 and 10 Ag). These glands 

 (Keber's organ) are supposed to be connected with the production of 

 the blood corpuscles. There are well developed valves between the 

 auricles and the ventricle. The valve is made up of two thickened 

 portions side by side {x and y). The muscles of the auricle attach to 

 these at their outer edges so that when the auricle is filled the lips 

 are pulled apart, leaving an opening between them. When viewed 

 from within the ventricle, one lip {y) is seen to have a cup-shaped 

 lid {z) attached to it. The ends of this lid are connected by muscular 

 fibres to the walls of the ventricle. When the chamber is filled the 

 lid is pulled toward the opposite lip, and fits down closely over the 

 opening between them. The walls of the ventricle are comparatively 

 heavy and form a close attachment with the part of the rectum pass- 

 ing through the pericardial cavity. The opening from the ventricle 

 into the aortic enlargement is on the left lower side. The aorta just 

 after leaving the heart is in the form of a rectangular chamber, with 

 an opening at each corner (Fig. K Ao). From it three important 

 vessels originate. The first on the anterior corner is the buccal aorta 

 {B.AÖ); the second on the outer angle opposite the ventricle is the 

 gastric aorta {G.Ao); and the third from the posterior angle is the 

 genital aorta {Gen.Ao). The buccal, or anterior aorta extends for- 

 ward along the dorsal wall of the visceral cavity to enter the buccal 

 sinus {B.s) on the right posterior side. Just before entering the 

 sinus, however, a very important artery is given ofif. This is the 

 palliai artery {Tal. a) which runs to the left in the anterior wall of 

 the visceral cavity, and at the point where the mantle cavity opens 

 on the left it divides, the smaller branch going around to the anterior 

 part of the mantle, the other larger branch extending back within the 

 wall of the visceral cavity almost beneath the border of the shell. 

 The terminal branches of this artery almost inosculate with those of 



