216 SYNOPTIC COLLECTION. 



later becomes ribbed or crenulated. In all the shells the 

 margin is entire. The adult of Helcioniscus has a circu- 

 lar foot, and the functional lamellar gills which are called 

 pallial gills lie in a groove between the foot and the man- 

 tle. These are really secondary gills, as the original 

 breathing organs exist as mere remnants on each side of 

 the neck. The heart of Helcioniscus consists of a single 

 auricle and ventricle. 



The eggs of Fissurella (PI. 439, figs. 1-12) are joined 

 together by an albuminous substance which swells in 

 water to a large size, like that in which frogs' eggs are 

 embedded. These eggs (fig. i) are not laid through the 

 apical hole, as has been supposed, but through the ante- 

 rior opening of the branchial chamber. The egg passes 

 through the usual stages of segmentation until the embryo 

 (fig. 2) with velum, beginning of foot, primitive mouth, 

 and invaginated shell gland is attained. 



While the embryo is still within the egg, the shell is 

 formed (fig. 2). At first it is made of particles of lime 

 separated from each other; afterwards these become con- 

 solidated, and the shell exhibits a lace-like pattern. Since 

 the characteristic twisting of the embryo has already 

 begun, the shell is asymmetrical, and the little embryo 

 has the characters of a young Gastropod, /. e., a coiled 

 shell, a ciliated velum, and a foot (figs. 3, 4). When 

 still older, the shell possesses an operculum (fig. 5). 



When the embryo leaves the egg, the last vestiges of 

 the velum remain and aid the foot in locomotion. The 

 embryonic shell is not replaced by another but is persis- 

 tent. Its margin begins to spread out, and the new 

 layers take on markings very different from the plain 

 embryonic shell (fig. 6, dorsal view; fig. 7, ventral side). 

 At this time the foot is long and narrow, and the long 

 tentacles have eyes at their bases. Up to this time the 

 margin has been entire, like the margin of Patella, but 

 soon a slit appears (fig. 8). Gradually this slit is sur- 

 rounded by shell layers whereby it is converted into a 



