PHYLUM ANNULATA 



485 



tent 



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 LumbricilS, and due to the FIG. 401. Anterior extremity of Sipun- 



,r P r culus nudus. ant. pan. anterior 



Same Cause VIZ., the presence [OI papillary repion ; post. pap. posterior 



two systems of intercrossing lines. ffiKaW' 1 ; '""' tentacular fol(L 

 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 

 cerebral num b er o f 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 



L<-I<;. 402. Tentacular fold of sipun 

 XT SS5?"wardT' r ' J ' 



