v PHYLUM PLATYHELMINTHES 285 



with the body- wall, and closed behind. Its wall in the eversible part 

 consists of an epithelium (internal when at rest) continuous with 

 the epidermis and similar to the latter, a basement membrane, 

 and either two or three layers of muscle, circular and longitudinal, 

 with an external thin epithelium of flat cells. The circular muscular 

 fibres are not continued back on the non-eversible part, but the 

 longitudinal fibres pass backwards to form the retractor muscle, by 

 means of which the proboscis is attached to the sheath in which 

 it is enclosed, and by means of which also it is retracted. The 

 internal epithelium of the proboscis develops variously formed 

 and arranged papillae, and in most cases its cells form rods of a 

 similar character to that of the rods or rhabdites of Turbellaria. 

 In the part between the eversible and non-eversible regions a part 



B f 



m,P ?-P /I 



PP 



FIG. 234. Diagrammatic representation of proboscis : (A) in the retracted condition, (B) in the 

 everted condition, g. p. glandular portion of the proboscis ; m. muscle attaching the 

 proboscis to its sheath ; TO. p. muscular portion of the proboscis ; p. p. in A, proboscis pore ; 

 p. p. in B represents the position of the proboscis pore in the retracted condition of the 

 proboscis ; p. s. proboscis sheath. (After Sheldon.) 



which may itself become elongated and complicated in structure- 

 is developed in many Nemerteans (Metanemertini) a median 

 calcareous stylet (Figs. 237, 238) with groups of smaller accessory 

 stylets at the sides. In the everted proboscis these are borne 

 at the free anterior extremity, and are thus capable of being 

 used as weapons. In Drepanophorus there are a number of small 

 stylets supported on a narrow curved plate, together with accessory 

 stylets. In the rest of the Nemerteans stylets are not developed. 

 It is by contraction of the muscular walls of the sheath, the 

 cavity of which (rhynchoccele) contains a corpusculated fluid, that 

 the proboscis becomes everted. The abundant nerve-supply of 

 the proboscis points to its being used partly as a tactile organ. 



The outermost layer of the body-wall is an epidermis of 

 columnar cells, manv of which are ciliated, while others are 



