ANATOMY AND PHYSIOLOGY OF ATRINA RIGIDA. 
435 
when the mantle is stimulated gently opposite the anterior end of the gills, the anterior 
part of the gills contract, while the posterior part of these organs remains quiet. A 
stronger stimulus causes the whole of the gills, as well as other parts, to contract. 
SENSE ORGANS 
OTOCYSTS. 
The otocyst in Atrina is located very near the tip of the foot and has therefore a 
very unusual position. (Fig. 12, ol.) It varies greatly in size in different specimens 
and may be a degenerating organ. In some specimens it was found to be an extremely 
Fig. 13. — Drawing of a transverse section of one lobe of the otocyst, 
outlined with a camera lucida. c. Ciliated tube connecting the 
otocyst with the exterior; e, ciliated epithelium forming the wall 
of the otocyst; o, otolith showing concentric structure. 
Fig. 14. — Reconstruction of the 
compound otocyst from a 
series of sections. 
small sac lined by ciliated epithelium containing no otolith. In others it is a large 
lobed structure with an otolith in each lobe. The otoliths (fig. 13, o) show a concentric 
structure. Figure 14 represents a reconstruction of the otocyst from a series of sections, 
and shows that in this specimen there were two or three otocysts in place of one, the 
usual number. There are three ciliated canals leading in toward the otocysts from the 
outside and although they could not be traced into the otocysts they came so near 
that there is scarcely an}' doubt but that they are the tubes formed by the invagination 
of the ectoderm, which gave rise to the otocysts. (Figs. 13 and 15, c .) The evidence 
indicates therefore that there are three otocysts in this specimen formed by independent 
