ON THE DEVELOPMENT OF LTXGULA ANATINA. 81 



The tentacle (PI. V., Figs. 77-79, and PL VI., Figs. 83-85, 

 tnt.), which is wanting in the larva of Cistella, appears to represent 

 one of the larval sense organs. It is usually shorter but broader 

 than a full grown cirrus and terminates bluntly. It contains a 

 central lumen (tentacular canal) (PI. VII., Fig. 120, tnt. en.) 

 which is loosely lined with epithelial cells. In some of these 

 cells the longitudinal muscle fibres can be detected. The muscle 

 I shall call the tentacular muscle (m. tnt.). It is through the 

 contractility of these muscle fibres that the shortening and bending 

 of the tentacle is effected. The tentacle is often bent elbow-like 

 at a place midway of its length (PI. V., Figs. 75, 77). The 

 wall of the tentacle (PI. VII., Fig. 121), as seen in a transverse 

 section, is composed of a thick epithelium, whose nuclei are on 

 the ventral side (v) similar to those of the cirri in shape, size 

 and affinity for stains, being, however, not so closely grouped 

 as those of the cirri. On the dorsal side (rJ), on the contrary, 

 there are arranged near the periphery one, or at the most a 

 few layers of vesicular nuclei, while in the more central part 

 the section presents a granular appearance corresponding to the 

 sections of nerve fibres. The above vesicular nuclei are certainly 

 those of sensory cells, while the compact nuclei situated on the 

 ventral side must be those of the ordinary ciliated epithelial cells. 

 Dorsal to the tentacular canal the supporting substance (PI. VIL, 

 Fig. 121, sp. I.) forms for a short distance a vertical plate which 

 at the outer end bifurcates into a pair of diverging septa ter- 

 minating among the epithelial cells. 



The epistome (Armfalte) (PI. VI., Fig. 85, epst.) which 

 made its first appearance at the 3 p. c. stage, has now become a 

 conspicuous shelf-like ridge, overhanging in front of the mouth. 

 It is a fold of the epithelial layer whose component cells do 



