452 



ZOOLOGY 



SECT. 



cer.org 



FIG. 353. Tentacular fold of Sipunculus 

 xmdus. cer. org. cerebral organ. (After 

 Ward.) 



the introvert are local thickenings of this cuticular layer. Beneath 

 the cuticle is an epidermis consisting 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 

 denned bodies. Of these there 

 are three kinds bicellular glands, 

 contained in papilla ; multicellu- 

 lar glands, scattered through the 

 integument, and not contained 

 in papillae ; and sense-papillae, 

 small rounded thickenings of the 

 epidermis in the anterior region 

 of the introvert, with their sum- 

 mits covered with cilia. There 

 are also numerous pigment cells. 

 Numerous canals branch through 

 the dermis. Beneath this 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 longi- 

 tudinal 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 ccelomic fluid are 

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

 ciliated bodies, the urns, which may be parasitic ciliate Protozoa, 

 but whose nature is obscure. 



The blood-vascular system is very feebly developed. It 

 consists of dorsal and ventral longitudinal sinuses (i.e. channels 

 differing from true vessels in wanting an internal epithelium), 

 communicating in front with a circular sinus at the base of the 

 tentacular fold. 



The alimentary canal (Fig. 354) is a cylindrical tube of uniform 

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

 back from the mouth towards the posterior end, and then bending 

 sharply round to run forwards to the anus, the two limbs being 

 twisted spirally round one another. Eunning along the entire 

 length of the alimentary canal, with the exception of the terminal 



