46 THE APODOUS HOLOTHTJRIANS 



tains is, in some species at least, entirely cut off from the tentacular canal 

 (Plate VI, figs. 19 and 20). At the tip of the tentacle the epithelium is much 

 thickened, especially on the outer side, and the thicke'ning probably serves as 

 a special organ of touch. There are no tentacle ampullae in any of the Syn- 

 aptidte. (See p. 147.) 



SENSE-OKGANS. The sense-organs of Synaptids are of four very distinct 

 sorts light-detecting, positional, gustatory, and tactile organs. The light- 

 detecting organs, or eyes, are known in only a comparatively few species, and 

 their function has been a matter of some dispute. They are best known in 

 Synaptula hydriformis, where at least they are simple eyes (Plate VI, fig. 15). 

 They are situated, a pair at the base of each tentacle, and consist of a distinct, 

 rather horny mesodermal layer, containing scattered nuclei, overlying the 

 swollen end of a large nerve, which arises on each side from the circumoral 

 ring at the base of the tentacle nerve. The ends of these nerves are made up 

 of nerve cells of considerable size with big nuclei, which are somewhat swollen, 

 and are apparently vacuolated at their outer extremities. They are polygonal 

 in outline when seen in cross-section, and the inner ends taper off into fibers 

 which run out into the nerve. The mesodermal covering, which also has the 

 appearance of being vacuolated, is clearly a continuation of the thin mesoderm 

 layer which surrounds all the nerves. The eyes are about 60 /u in diame- 

 ter, the mesodermal covering being rather more than one-tenth as thick. The 

 pigment, which is contained in the mesodermal covering, is at first bright green, 

 but gradually becomes deep reddish brown, so that the eyes look like small 

 brown spots. In some instances it has been shown that the so-called "eye- 

 spots" are simply clusters of "wandering cells" and are not supplied with 

 special nerves, and are therefore of course not sense-organs at all. The posi- 

 tional organs are generally, though incorrectly, called "auditory vesicles," or 

 "otocysts." They are known to occur in a number of species of Leptosynapta, 

 Synaptula, Protankyra, Labidoplax, Chiridota, Myriotrochus, and Acanthotro- 

 chus, and no Synaptid is known to lack them. They are spherical sacs situated 

 just outside the calcareous ring, one on each side of each radial nerve, with 

 which they are connected by small nerves. They differ greatly in size, those 

 of Synaptula hydriformis (Plate VI, fig. 16) measuring only 60 or 70 /* in diame- 

 ter, while those of Labidoplax digitata are sometimes as much as 210 /*. They 

 are lined with cilia and filled with fluid, and contain one or more little spheres, 

 which are kept in active movement by the action of the cilia. These spheres are 

 heavier than the fluid in which they lie, and constantly tend to sink to the lowest 

 part of the sac, and by contact with the cilia give rise to sensations of changed 

 position. Whether these spheres are made up wholly or in part of inorganic 

 salts is still undetermined, but they appear to be vesicles filled with a denser 

 fluid than that in the sac. The gustatory organs (Plate V, fig. 22 ; Plate VII, figs. 



