OUR KNOWLEDGE OF NEW ZEALAND HOLOTIiURIANS. 119 



thickness and finally disappearing at the end of the pinnules, (v.) A layer 

 o£ peritoneal cells. 



JS^ervous System. — The central nervous system consists o£ a ring o£ nervous 

 tissue surrounding the ]3harynx between the latter and the calcareous ring. 

 The nerve-cells are arranged in definite bands and are thus very conspicuous, 

 as they stain deeply. From the nerve-ring arise five radial nerves (figs. 17, 

 2'd, n.) which run forwards between the tentacular canals, bend outwards 

 beneath the five longitudinal muscles and then run backwards beneath the latter 

 for a very short distance. They disappear before reaching the level of the 

 calcareous ring^ leaving only a cavity, containing some structureless sub- 

 stance which looks as if it were a coagulum (fig. 22, cav.). This cavity probably 

 represents both epineural and hyponeural canals from which the nerve has 

 disappeared. It persists to the posterior end of the body, where it ends 

 blindly. 



The structure of the radial nerve before it disappears is quite typical 

 (fig. 21). Each is divided into two bands, an inner and an outer, by a very 

 thin septum, which passes across the nerve nearer to the hyponeural than the 

 epineural canal. The nerve-cells are arranged chiefly in two aggregations on 

 the sides of the outer band, as shown in the figure. There is also a tract of 

 nerve-cells on the inner side of the inner nerve-band adjacent to the hypo- 

 neural canal. The epineural canal is large and well-defined, the hyponeural 

 less so. 



Each radial nerve gives off a pair of nerves shortly after its origin ; each 

 of these nerves supplies the corresponding otocyst. 



In addition to the nervous system described above, there is the continuous 

 layer of nerve-fibres and nerve-cells immediately exterior to the circular 

 muscular layer of the integument. This layer supplies nerves to the sense- 

 papillse and to the " taste-pits," described below. 



Sense- Organs. 

 (i.) Sensory papillce (fig. 22, p.). — These are distributed over the whole 

 surface of the body, but are most numerous in the anterior part. Each 

 consists of a core of connective tissue covered by the epidermis, and 

 contains a special ganglionic mass ((/.), supplied by a nerve (n.jj.) which 

 comes from the nerve-layer. The ordinary columnar cells of the epidermis 

 in these papillae are much longer than those of the general surface of the 

 body. The ectoderm also contains gland-cells and sensory-cells similar to 

 those described by Hamann in the sense-papillse of Synapta '^. The lower 

 ends of these sensory-cells are connected with the ganglion by means of 

 nerve-fibrils, which form a thin layer beneath the ectoderm-cells, and appear 



* Hamann, Beitrage zur Histologie der Echinodermen. Die Holothurien, p. 18. 



