DEVELOPMENT OF MOLLUSCS. 1 99 



arrangement. This gemmation, or, in other words, this 

 stem structure, still occurs in Echinoderms, inasmuch as 

 some species of star-fish possess such powers of repro- 

 duction as to enable a single arm or ray, when torn off, to 

 complete itself into a whole animal. Nay, Kowalewsky's 

 observations render it highly probable that the separa- 

 tion of rays, and their completion by gemmation, is in 

 some species a normal process. Haeckel's hypothesis 

 is thus laughed at only by those who are afr.aid to think 

 or reason. 



In the famiily of the Mollusca, the so-called navicula 

 larva testifies the kinship of at least two of the great 

 classes. The third and most advanced class, that of 

 the cuttle-fish, had perhaps lost their distinctive badge 

 even in those primaeval times when, under the somewhat 

 lower forms of the Tetrabranchiata, they left their shells 

 in the Silurian strata. But the bivalve shells, or Lamelli- 

 branchiata, and the snails, widely differing in anatomical 

 development, and constituting two natural classes, have 

 a common larval form, or, if the larvae display different 

 shapes, a highly distinctive common larval organ, the 

 velum. The accompanying diagram gives on the right 

 the navicula of a cockle-shell as seen from behind. At 

 the anterior end, two fleshy lobes have been formed, edged 

 with cilia, by the vibrations of which the young animal, 

 even in the egg, performs spiral twisting motions ; in 

 the midst of the cilia rises a little prominence, furnished 

 with a longer filament. These ciliated lobes or vela, 

 merging into one another, are shown on the left in the 

 larva of a sea-snail (Pterotrachea), as seen nearly in 

 profile, and in the phase in which the eyes and auditory 

 apparatus, the foot and operculum, as well as a delicate 



