the cerebral ganglia, but a little ventral, anterior and nearer the 

 median plane of the body than these ganglia. The otocysts are 

 imbedded in a mass of connective tissue that surrounds the pedal 

 ganglia, and may with comparative ease be dissected out with 

 the pedal and the cerebral ganglia, and studied in total mounts. 



Each otocyst consists of a nearly spherical pouch formed of 

 epithelial cells that is connected with the exterior by a small 

 canal (fig. 24, otc.) that opens almost opposite the cerebral gan- 

 glion of the corresponding side. Similar canals are present 

 either as complete canals or as rudiments in different species 

 of the Protobranchia, but so far as I know, have never before 

 been described for any species outside of this group. As oto- 

 cysts uniformly originate as invaginations from the surface 

 epithelium of the animal possessing them, it is reasonable to 

 •uppose that these canals are simply persistent from the embryo- 

 logical condition, but this has not yet been proved. The oto- 

 cysts are ciliated (whether the cilia are vibratile or not has not 

 been observed) and usually contain a considerable mass of fine 

 granular material that may be scattered, or collected into a very 

 definte ball. It is not at all uncommon to find one otocyst nearly 

 filled with this material, while the other is nearly empty. On 

 the other hand, both may be nearly filled, or both may be nearly 

 empty. The origin of this material is doubtful. In appearance 

 it resembles fine fragments of debris such as is found on the 

 bottoms where the scallops live, and there is sometimes some 

 variation in the color of these particles. All of the particles 

 seem to be sufficiently small to have been introduced through 

 the otocystic canals, but I have not thus far found any of the 

 shells of diatoms although the mud on which the animals live 

 is full of them and many of them are as small as the particles 

 that are found in the otocysts. The irregular, broken appear- 

 ance of the particles and the fact that frequently there are many 

 more particles in one otocyst than in the other rather points to 

 their being foreign particles than to their being products of 

 secretion. 



The otocysts are usually considered to be static organs. So 

 far as I know, there are no experiments that bear on the func- 

 tion of these organs in lamellibranchs, and the supposition that 

 they are organs for determining position in space is based upon 



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