PHYLUM ANNULATA 



498 



Fig. 887 



Tentacular fold of Sipunculus 

 nudus. cer. onj, cerebral organ. (After 

 Ward.) 



Body-wall. — The surface is covered by a thitinoid cuticle 

 having an iridescent lustre similar to that presented by the cuticle 

 of Nereis and Lumbricus, and due to the same cau.se — viz., the 

 presence of two systems of intercrossing lines. The papilla; on 

 the introvert are local thickenings 

 of this cuticular layer. Beneath 

 the cuticle is an epidermis con- 

 sisting of a single layer of cells, 

 usually sac-like, but capable of 

 being altered as a result of con- 

 traction or compression into a 

 spindle-like shape. Below the 

 epidermis is a layer of connective- 

 tissue, the dermis, in which, as 

 well as to some extent in the 

 epidermis itself, are a number 

 of dermal todies. Of these there 

 are three kinds — hiceUular glands, 

 contained in papillae ; mulii- 

 cellular glands, scattered through 

 the integument and not contained in papillse ; and sense-pcqnllo', 

 small rounded thickenings of the epidermis in the anterior region 

 of the introvert, with their summits covered with cilia. There 

 are also numerous pigment-cells. A number of canals branch 

 through the dermis, beneath which are three layers of muscle — 

 (1) an outer circular layer, continuous in the introvert, but 

 divided into annular bands in the rest of the body ; (2) an oblique 

 layer, well developed only between the origins of the two retractor 

 muscles of the introvert; (3) a lo7igitudinal layer, which is 

 separated by spaces into a series of parallel bands. Between the 

 bundles of the longitudinal layer of muscle runs a series of canals 

 which communicate with the body-cavity by transverse branches. 



There is a spacious ccelome, but it is traversed in all directions 

 by filaments and strands of connective-tissue, with which are mixed 

 very fine muscular fibres ; these mostly run from the wall of the 

 body to the alimentary canal. Floating in the cuslomic fluid are 

 (1) colourless corpuscles; (2) reproductive elements; (3) peculiar 

 ciliated bodies, the urns, which are developed by proliferation 

 from cells on the wall of the dorsal blood-vessel. These 

 are comparable in structure and function with the ciliated funnels 

 of the Hirudinea (q.v.). 



The blood-vascular system is very feebly developed. It 

 consists of dorsal and ventral contractile vessels, the former known 

 as the "heart," communicating in front with a circular sinus 

 at the base of the tentacular fold. 



The alimentary canal (Fig. 388) is a cylindrical tube of uniform 

 character throughout. It is twice the length of the body, running 



