PHYLUM ANNULATA 



493 



FIG. 3S7. Tentacular fold of Sipunculus 

 nudus. ccr. or<j. cerebral organ. (After 

 Ward.) 



Body-wall. The surface is c^ered by a chitinoid cuticle 

 having an iridescent lustre similar to that presented by the cuticle 

 of Nereis and Luinbricus, and due to the same cause 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 bodies. Of these there 

 are three kinds bicellular glands, 

 contained in papilla? ; multi- 

 cellular glands, scattered through 



the integument and not contained in papilla?; and sense-papilla', 

 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 longitudinal 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 ccBlome, 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 ccelomic 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 



