198 SUMMAEY OF CURRENT RESEARCHES RELATING TO 



symmetrical about itself, and on each side of it are from 25 to 32 teeth, 

 so that there are in all 51 to 65 longitudinal rows. 



In Velletia the radula has some 84 transverse rows of teeth. The 

 well -developed central tooth is smaller than in Ancylus; 11 to 13 teeth 

 lie on each side of it, and outside these come 4 to 7 little plates, the row 

 thus containing 35 to 37 pieces. Comparing these with the mouth- 

 organs of other members of the family, Herr Ulicny concludes that a 

 natural classification demands the breaking up of the family Limnseidae 

 into several independent families. 



8. Lamellibrancliiata. 



Edge of Mantle of Acephala.* — Dr. B. Rawitz deals, in his first 

 account of the mantle of acephalous Lamellibranchs, with the Ostreacea. 

 In Anomia epJiippium the margin forms a thickening of the mantle which 

 is best developed in the middle, and gradually passes to a simple ridge 

 at the edges. The left margin of the mantle is feebler than the right ; 

 its inner surface is generally flat, and has only rarely an elongated 

 brownish swelling which is always at some distance from the edge ; the 

 inner surface of the right half has a rounded swelling of the tissue 

 which, though varying with the age of the animal, is always better 

 developed than that of the left. Both edges are beset with several rows 

 of conical papillse or tentacles, of which the innermost are the longest 

 and the outermost the shortest. This arrangement obtains also in Ostrea, 

 Lima, Spondylus, and Peden. 



The edge of the mantle of Ostrea edulis is pigmented at the centre of 

 its curve, but is free of pigment on the dorsal and ventral sides of the 

 animal. As in Anomia the two halves of the mantle are separate along 

 their whole extent ; macroscopically, these two halves are exactly 

 similar. 



In Lima the tentacles are all marginal in position ; the mantle- valve 

 is extraordinarily long, as is, in the Spondylidas and Pectinidse, the 

 valve which hangs down inwardly ; in the latter, however, its free 

 margin may carry several rows of short tentacles. 



The nerves which supply the mantle and its edge, arise from the 

 cerebral and visceral ganglia. Each of the former gives off one nerve, 

 the nervus pallialis anterior ; it divides dichotomously into two branches 

 which fork and end in very fine branches in the substance of the margin ; 

 it is principally supplied to the anterior fourth. The median pallial 

 nerve arises as a strong trunk from the visceral ganglion, and has at first 

 a direction perpendicular to the long axis of the animal ; at the margin 

 it divides into two equal branches which diverge and supply a large part 

 of the edge with finer branches. The hinder pallial nerve is relatively 

 delicate, and divides into three fine branches which supply the greater 

 part of the hinder fourth of the margin. The finest branches of this 

 nerve supply pass into a nervus pallialis circularis. 



The differences in the minute structure of the mantle and its margin 

 in the five families of Ostreacea which were examined are so great that 

 the author describes each separately ; this he does in greater detail than 

 we are able to follow him. But some points of interest may be noted. 



In the Anomiidse the epithelia which cover the marginal tentacles 

 in several rows are sensory or indifferent ; the latter may or may not be 



* Jenaische Zeitschr. f. Naturwiss., sxli. (1888) pp. 415-556 (6 pis.). 



