414 GEPHYREA CHAP. 



The fringe may be in the form of a simple ring round the mouth, 

 or the ring may be folded in at the dorsal side so as to take 

 the form of a double horse-shoe (Figs. 211 and 212). 



Body -wall. The glistening appearance of Sipunculus is 

 due to the cuticle, a chitinoid layer which is secreted by the ex- 

 ternal layer of cells, the epidermis. Beneath this lies a layer of 

 connective tissue, which is not always present in other Gephyrea ; 

 within this lies a layer of circular muscles arranged in bundles, 

 then comes a very thin sheath of oblique muscular fibres, then 

 a thicker layer of longitudinal muscles, and finally a layer of 

 peritoneal epithelial cells, which in Sipunculus are for the most 

 part ciliated. 



Scattered over the surface of the body, and opening by nar- 

 row tubes which pierce the cuticle, are a number of glandular 

 bodies which may be either bi- or multi-cellular. The glandular 

 cells are apparently enlarged and modified epidermal cells ; they 

 are arranged in a cup-shaped manner, with their apices directed 

 towards the orifice. They are crowded with granules, which 

 are presumably poured out over the cuticle, but the exact 

 function of the secretion is entirely unknown. They have a well- 

 developed nerve supply. 



Digestive System. The mouth lies in the centre of the 

 fringe, and is not provided with any kind of jaw or biting arma- 

 ture; it leads directly into the thin- walled alimentary canal, the 

 first part of which is ciliated. The alimentary canal is not 

 marked out into definite regions, but passes as a thin -walled 

 semi-transparent tube to the posterior end of the body, and then 

 turns forward again and opens to the exterior by an anus situ- 

 ated about an inch below the junction of the introvert with the 

 trunk, on the median dorsal line. The descending and ascending 

 limbs of the alimentary canal are coiled together in a spiral, 

 which may be more or less close in different individuals. The 

 whole is supported by numerous fine muscular strands, which 

 pass from the walls of the intestine to the skin, and by a spindle- 

 muscle, which runs from the extreme posterior end of the trunk 

 up the axis of the spiral and terminates in the skin close to the 

 anus. 



No glands open into the alimentary canal at any point of its 

 course, but near the anus a simple diverticulum, or pocket, of 

 unknown function arises. The size of this outgrowth differs 



