PHYLUM ANNULATA 



485 



post /bap 



FIG. 401. Anterior extremity of Sipun- 

 culus nudus. ant. pap. anterior 

 papillary region ; post. pap. posterior 

 papillary region ; tent, tentacular fold. 

 (After Ward.) 



longer and narrower, mammilliform, and more scattered. When 



the introvert is fully evaginated, there appears at its extremity 



a horse-shoe-shaped fold of the integument, the tentacular fold 



(tent.), which is lobed and plaited 



(Fig. 402) so as to assume somewhat 



the appearance of a circlet of 



tentacles. For a little space imme- 

 diately behind the tentacular fold 



the surface of the introvert is free 



from papillae. The posterior portion 



of the body is devoid of papillae, 



but is marked out by a series of 



narrow impressed lines into a 



number of elongated four -sided 



areas. 

 Body- wall. The surface is 



covered by a chitinoid cuticle having 



an iridescent lustre similar to that 



presented by the cuticle of Nereis 



and Lumbricus, and due to the 



same cause viz., the presence 'of 



two systems of intercrossing lines. 



The papillae on 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 contraction 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 papillae ; multicellular 

 glands, scattered through the in- 

 tegument and not contained in 

 papillae ; and sense-papillce, small, 

 rounded thickenings of the epider- 

 mis 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 



FIG. 402. Tentacular fold of Sipun- 

 culus nudus. cer. org. cerebral 

 organ. (After Ward.) 



