78 HUBERT LYMAN CLARK ON 



(Figs. 81-83), already mentioned as lying external to the radial nerves at the point where 

 they bend backwards over the calcareous ring. They are much smaller than those figured 

 by Cuenot ('91) for S. inhaerans and differ from them in having only a single large 

 vesiculated cell enclosed within them, instead of a number of small ones. The otocysts 

 of S. vivipara measure only about 60-70/1 in diameter, while the contained cell is almost a 

 quarter as large. In no case have I found more than one cell enclosed in an otocyst of 

 this species. Hamann ('84) suspected that they were larval organs having no function 

 in the adult, but Semon ('87) has already proved that idea erroneous, as Hamann ('89, 

 p. 308) has since admitted. If any further evidence were needed, it could be found in 

 the increase of size of the organs during the development of the animal (Figs. 81-83) as 

 well as in the very obvious connection with the radial nerves. But I am inclined to the 

 view that these so-called otocysts do not function as hearing organs at all, but are of use 

 to indicate the animal's position. Semon ('87) was unable to find any cilia in them, and 

 his experiments on living Synaptas brought him to the conclusion that they were deaf to 

 sound waves. If the enclosed cell is vesiculated, as it appears to be, it must float in the 

 fluid with which the otocyst is filled and so presses on that part of the wall which is 

 uppermost. Any change in the position of the animal would cause a corresponding 

 change in the position of the enclosed cell and thus give rise to a changed sensation. 



The fully grown Synapta mvipara (Fig. 20) measures from ten to fifteen centi- 

 meters in length and from four to nine millimeters in thickness ; the size depending 

 largely on the state of contraction of the muscles. In color they vary from a pale 

 reddish brown to a very dark greenish brown more or less spotted and blotched with 

 white. The ground color is due to the pigment in the connective tissue of the body-wall 

 and varies greatly with the amount of that pigment, but the white spots and blotches are 

 due to the aggregation of great numbers of the miliary granules, just beneath the 

 ectoderm. The pigment is not affected to any extent by pure alcohol, but corrosive 

 sublimate and all acids destroy or greatly modify it. Just posterior to the calcareous 

 ring and in connection with it there is a ring of cartilage-like connective tissue (Fig. 

 90). This structure was described and figured by Theel ('86), who also figured the 

 anchors and plates from the body-wall, in his account of S. picta. The anchors (Fig. 51) 

 lie close under the ectoderm and parallel with it, at right angles to the main axis of the 

 body. Each anchor is much curved or bowed inwards, while its arms or flukes are curved 

 outward so that the points of the arms are always projecting. The vertex is not toothed 

 but has five or six almost spherical knobs on its edge. The posterior end is broadened 

 out into several short, very finely-toothed branches. Beneath the anchors lie the rounded, 

 smooth-edged, somewhat arched plates, which normally possess seven large toothed holes 



