STRUCTURE AND DEVELOPMENT OF LOXOSOMA. 273 
ectodermic sense-cells may be seen either in the living animal 
or in glycerine preparations in a manner which leaves room for 
no doubt as to the actual existence of this arrangement. 
It remains now to consider the structure of the two posterior 
sense organs of L. crassicauda. In silver preparations 
(fig. 4) each is seen to consist merely of an exaggerated sense- 
cell, bearing a number of hairs, and provided with a large 
nucleus. The sense-cell passes into a nerve continued into a 
large number of ganglion-cells, instead of one only, as in other 
regions of the body of this species. The increase in the 
number of these cells is no doubt correlated with the speciali- 
sation of the sense organ. The arrangement in L. pes indi- 
cated in fig. 6, forms an intermediate condition between the 
two kinds of sense-cells of L. crassicauda. The posterior 
sense organs of the latter differ from the sense-cells borne on 
other parts of the body in forming distinct papillae, at the 
apex of each of which is an enlarged sense-cell. In most 
cases (fig. 6) three of the flat epidermic cells are concerned in 
the formation of the papilla. 
In Pedicellina no special attention was given to the 
peripheral nervous system. The ganglion ( ga .) is, however, 
shown in fig. 12, a horizontal section ; it consists of a central 
fibrous mass and of peripheral ganglion-cells. Instead, 
however, of being greatly elongated transversely, as in 
Loxosoma, it is oval; at the sides come off several pairs of 
nerves. 
In a picrocarmine-glycerine preparation of Pedicellina, 
each tentacle (fig. 5) is traversed by a fine nerve, which gives off 
branches on its outer side, each passing into a bipolar ganglion- 
cell connected with an ectodermic sense-cell ; one of the latter 
appears to be provided with a sense hair. 
In the larva of Pedicellina, Hatschek (14) has called 
attention to the existence of tactile hairs on various parts of 
the body (Taf. xxix, fig. 26). These have exactly the appear- 
ance of the sense hairs of the adult Loxosoma, but their 
connection with the nervous system has not been observed. To 
the same category probably belong the stiff hairs arranged in a 
