1883.] NATURAL SCIENCES OP PHILADELPHIA. ♦ 219 



■which the blood-spaces (lacunae) are found. A long continuous 

 one runs the whole length of the inferior border of the gill, and is 

 in connection with the mantle-vein. The nuclei of the connective- 

 tissue fibres are very distinct ; the rectum passes perpendicularly 

 through the tissues of the middle of the gill, and opens at the 

 anus, situated on the external surface. 



Several organs open into the branchial chamber ; in the middle 

 of the external surface of the gill, as said, opens the anus. In 

 A. la custris, when the gill is on the right side the rectum iind anus 

 also are on that side. Close behind the base of the left tentacle in 

 A.Jluviatilis, is found the male genital pore or opening, and close 

 behind this the female ; as with the anus, these openings are on 

 the right side of A. lacustris ; in A. fiuviatilis, on the internal 

 surface of the left mantle, is found the minute opening of the 

 excretory organ, the kidney, which lies embedded in the tissues of 

 the mantle ; in A. lacustris the kidney is on the right side ; thus 

 we see that four organs open into the branchial chamber, the S 

 and 9 genital openings, the anus, and the kidney. 



The alivientary canal. — The mouth, which opens on the inferior 

 surface of the body, is surrounded by three lips ; the two anterior 

 lips are placed together so that they form an inverted V (A) ; the 

 open part of the Y is closed by the under lip, which is the extreme 

 anterior end of the sole of the foot. 



The mouth leads into a small tube, which passes perpendicularly 

 upwards, opening on the floor of the buccal mass (Plate X,fig. 2jji). 

 About half-wa}' between the mouth and the buccal mass is situated 

 the horseshoe-shaped jaw, which is placed in the anterior wall of 

 the tube. The jaw consists of a single membrane of conchyolin, 

 upon which are situated numerous little teeth or denticles. 

 Moquin-Tandon says, however, that '■'' Ancylus possesses three 

 jaws, 'disposed as those in Limnseua — ^a transverse one above, and 

 two vertical ones on the sides, * * * the borders of which 

 are formed of a series of little denticles." ^ I do not find this to 

 bev exactly the case, but agree with Keferstein, who saj'S : " In 

 Ancylus we see, instead of the simple jaw, a large number of long 

 pieces, which are tolerably symmetrically arranged, and encircle 



' L* Ancyle possedfe trois machoires, disposees comme celles des Limnees, 

 uue transversale, en liaut et deux verticle, sur le cotes * » * celles des 

 bords formeut comme une serie de petites deuticules : Recbe. anat. 

 physiol. s, TAncyle, etc., p. 16. 



