ANATOMY OF ACLESIA FREERI. 73 



the color there is lighter than on the surrounding parts. The 

 gland appears to secrete large quantities of mucus, but no dye 

 seems to be produced there, nor does the secretion have a dis- 

 agreeable odor, as in some other aplysiids. 



INTERNAL ANATOMY. 



The osphradium lies in the angle under the extreme anterior 

 end of the base of the branchia (text figure 1). It has the 

 form of an oval papilla, only slightly raised above the surround- 

 ing surface; it is depressed a little at the center, where the 

 osphradial ganglion is in contact with the dermis. 



The pericardium is thin-walled and occupies the space between 

 the mantle rim and the anterior wall of the respiratory chamber. 

 It is bounded posteriorly and overlapped by the nephridium, 

 which is light-colored and somewhat elevated. The lateral 

 (left) border of the kidney is approximately parallel to the 

 mantle rim, although in most cases the posterior end of the 

 kidney is much narrower than the anterior. The external aper- 

 ture of the kidney, the reno-branchial pore, is found on a 

 thickened and pigmented area at the posterior extremity of 

 the mantle rim, to the left of the base of the anal papilla. The 

 anterior and lateral portions of the nephridium are composed 

 of solid glandular substance. The postero-median region con- 

 tains but little glandular tissue, which lines the walls, while the 

 central part is occupied by a more or less distinct cavity. The 

 lateral edge of the nephridium lies over the large vein which 

 collects blood from the left side of the body. Numerous minute 

 openings are visible in the surface of the nephridium which 

 lies against the vein. The pericardium and nephridium lap to 

 such an extent that nearly one-half of the latter overlies the 

 pericardium. The reno-pericardial pore is formed at the ex- 

 treme posterior end of the pericardium, close to the angle 

 formed by the dorsal, posterior, and median walls. 



Veins from the nephridium open directly into the left side of 

 the auricle. 



ALIMENTARY SYSTEM. 



The narrow, slit-like mouth lies in the middle of a nearly 

 circular, flat, oral plate, the thickened skin of which is raised in 

 radiating, pleated ridges (fig. 10, Plate III). The flattened, 

 free edges of the ridges are white, while the grooves between 

 them are flecked with black spots. The ventral edges of the 

 oral lobes are continuous with the surface of the oral plate. 

 Just within the lips, each side of the buccal cavity is armed by a 



