

18 W. K. BROOKS ON THE 



Fig. 20, plate 3, is the embryo of a fresh-water Puhiionate, in what I regard as a 

 similar position. The foot,/, is below ; the shell, sh, above ; the month, m, the tentacles, t, 

 and the head on the right, and the rectum, r, on the left. The figvire therefore shows the 

 right side of the body of the Gasteropod ; and as the twist by which the rectum and the 

 mantle-chamber are subsequently carried around the right side of the body on to the 

 anterior surface has not yet appeared, the embryo is bilaterally symmetrical, and the 

 mouth, the anus, the foot, the mantle, and the shell occupy substantially the same positions, 

 and bear about the same relations to each other, that they do in a Lamellibranch. A 

 comparison with fig. 19, or with fig. 10, will show that there is, at the same time, an 

 essential similarity to a Cephalapod. 



In the GasterojDod, fig. 20, or in a Lamellibranch, we have the bilaterally symmetrical 

 shell, sh, resting like a cap upon the dorsal surface of the body, and surrounded by a 

 reflected ridge of integument, the margin of the shell area, sa. 



As Eay Lankester has pointed out, the embryonic shell of a Cephalopod is very similar, 

 and in the Squid, fig. 19, we have the external shell, sh, surrounded by the reflected rim 

 of integument, sa, exactly as in the Pulmonate. 



Running around the shell and shell area in both Cephalopod and Gasteropod, is a 

 second ridge of integument, the margin of the mantle, ma, ma, which already' j^rojects a 

 little from the posterior, ventral surface of the body of the Cephalopod, to form the outer 

 wall of the rudimentary mantle chamber. 



On the middle line of the posterior surface of the body, just below the mantle-ridge, 

 in both Cephalopod and Pulmonate is the rectum, r, with its external opening, the anus ; 

 raised from the surfoce of the body upon the anal papilla in the Cephalopod, but other- 

 wise alike in the two forms. 



On each side of the rectum of the Cephalopod is a single tentacular gill, g, underneath 

 the overhanging edge of the mantle. There are no corresponding structures in the 

 Pulmonate, and in the Prosobranch the gills do not make their appeai'ance until the 

 symmetry of the body has been distorted by the twisting of the visceral mass into a 

 spiral, but the gill filaments of a Lamellibranch are exactly similar in form, structure and 

 position to the gills of the Squid embryo. 



On the anterior surface of the body, the surface which is usually called dorsal in the 

 Cephalopod, and which is on the right in both figures, we have first, the thickened mantle 

 ridge, ma, which forms the angle between the dorsal and the anterior surface. Next we 

 have in the Pulmonate and in most Gasteropods the large pulsatile '■ neck region," h. In 

 the Cephalopod this region is short, not pulsatile, and it forms the " back " of the animal. 



Below the neck region of the Pulmonate a tentacle, t, is developed on each side of the 

 median line of the body, and the eyes subsequently make their appearance upon these 

 tentacles. On each side of the corresponding region of the body of Squid embi-yo, the 

 projecting eye-stalk, t, carries the rudimentary eye upon its rounded extremity, and there 

 does not seem to be any doubt that the sensory tentacles of the Gasteropod, and the 

 eye-stalks of the Cephalopod are strictly homologous. 



In the Pulmonate the rudimentary velum, v, is marked by a line of granular ciliated 

 cells, which crosses the middle line of the body just below the tentacles, and then run- 

 ning out upon the sides, bends up towards the dorsal surface, in such a way as to almost 

 encircle the tentacles. 



