DEVELOPMENT OF MOLLUSCS. 199 
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 rey, when torn off, to 
complete itself into a wholeanimal. 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 afraid to think 
or reason. 
In the family 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 primeval 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 larve 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 
