3. THE INTEGUMENT AND ASSOCIATED STRUCTURES 



37 



above (p. 18), and, as already stated, each platelet is the head, as it were 

 expanded and appearing to lie on the surface of the body, of a long bristle. 

 This broadened terminal plate-like head is attached to a long narrow stalk, 

 penetrating deeply into the thick epidermis of the girdle (Jagersten, 1956). 

 In the majority of species the greater part of the sharp teeth on the surface of 

 the head of the bristle point forwards (Figs. 13, 26B), indicating that the 

 function of the toothed platelets is to anchor the trunk firmly to the walls of 

 the tube at the moment of rapid withdrawal of the animal. The head of the 

 bristle, i.e. the platelet, consists of a solid yellowish transparent substance 

 without noticeable structure, and, as may be seen after several staining 

 methods, rather sharply delimited from the stalk. The colourless stalk, 

 displaying thin longitudinal striations, lies directly between epidermal cells. 

 Its external surface is formed of a very thin layer, which stains in the same 

 way as the cuticle of the epidermis. To the inner end of the stalk is attached a 

 conical cell transpierced by fine tonofibrillae which bind it to the basement 

 membrane of the epidermis. There is neither a particular socket for the 

 bristle nor a special musculature connected to the stalk (Fig. 26). Jagersten 

 (1956), however, in describing the toothed platelets of Siboglinum ektnani y 

 wrote that the stalk plunged into a special cavity of the epidermis and was 

 furnished at its inner end with muscle fibres. 



-cut 

 -ep 



Fig. 26. Epithelium of the girdles and the toothed bristles of Lamellisabella zachsi. A - in transverse 

 section ; В - in longitudinal section. 



cap - cap over a developing toothed bristle ; cb - basal cell of a bristle ; cch - cell which forms a bristle ; 

 «/(-epidermal cell; ch - stem of bristle; ch 1 , ch 2 - young developing bristles; cut- cuticle; 

 ep - epidermis ; fm - muscle fibres ; mb - basement membrane ; pd - toothed platelet head type of 

 bristle; vac - vacuole around a young bristle. (After Ivanov, 1960a.) 



