OUR KNOWLEDGE OF NEW ZEALAND HOLOTHUKIANS. 115 



three parts when irritated. Of the spirit-specimens in our possession about 

 half the number exhibit two very well marked constrictions, one about the 

 middle of the body, and the other about midway between this and the 

 posterior end. The constriction in the middle of the body is of very constant 

 occurrence, for out of about thirty specimens only one (fig. 1) was entirely 

 free from it. There does not appear, however, to be any special structural 

 modification in relation to this tendency. 



Integument. — The body- wall, as in all known Holothurians, exhibits four 

 layers: — (i.) an epithelial layer (epidermis) ; (ii.) a connective-tissue layer 

 with a nervous layer internally ; (iii.) a muscular layer ; and (iv.) a peri- 

 toneal layer. 



(i.) Epidermis. — A thin structureless cuticle (fig. 4, cm.) covers the outside 

 of the epidermis and appears to be continuous over the whole surface of the 

 animal. The epithelial cells (figs. 3, 4, 22, c.e.) are elongately columnar and 

 arranged in a single layer; the average size of each is about 15 /x. by 3 /a. Each 

 cell possesses a conspicuous deeply-staining nucleus, which is usually situated 

 about the middle of the cell. All the cells taper internally and become more 

 granular. 



In addition to the ordinary columnar epithelial cells, gland-cells (fig. 22, 

 gli) occur, especially on the tentacles and on the papillse of the integument. 

 They are elongated ovoid cells, each possessing very granular protoplasm and 

 a large nucleus. Sense-cells of the usual type also occur in the pajDillse ; 

 they will be described later under the head of sense-organs. 



(ii.) Connective-tissue layer (figs. 3, 4, 22, c./.). — This usually consists of a very 

 loose layer of connective-tissue fibres containing a few nucleated connective- 

 tissue cells scattered about amongst them. It varies considerably in thickness 

 (from 5 /a-50 /a), being especially thick in the sense-papillse. On the inner 

 face of the connective-tissue layer is a layer of nerve-fibres and nerve-cells 

 (n.Z.) In the sense-papillse there is also a thin layer of nerve-fibres (fig. "22, 

 s.ni) just beneath the epidermis, which is connected with the deeper nervous 

 layer by a special nerve [n.p.) in each papilla. This layer does not seem to 

 occur in other parts of the body. 



(iii.) Muscular layer, — There is a layer of circular muscles (figs. 3, 4, 22, cm.) 

 immediately beneath the nerve-layer. Its average thickness is about 10 fx. 

 This layer is continuous, not being interrupted by the longitudinal muscle- 

 bands. At short intervals throughout the length of the body it becomes 

 thickened and forms a series of transverse ridges projecting into the body- 

 cavity. 



The longitudinal muscles form five stout radially situated bands arising from 

 the calcareous ring and running back to the posterior end (figs. 16, 22, l.m.). 



(iv.) Peritoneal layer (fig. 22, per.). — This is a single layer of cubical cells 



