PHYLUM PLATYHELMINTHES 



Ir 



7TL 



of the most characteristic features of this class of Worms. The 

 proboscis is hollow ; when extended to its utmost, a part still 

 remains which is not capable of being everted. This hollow tube 

 (Fig. 229) is open in front, where its edges are continuous 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 pro- 

 boscis 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. 

 Exceptionally the cells contain 

 nematocysts similar to those of 

 Coelenterates. In the part be- 

 tween the eversible and non- 

 eversible regions, a part which 

 may itself become elongated and 

 complicated in structure, is de- 

 veloped in many Nemerteans 

 (Metanemertini) a median cal- 

 careous stylet (Figs. 232, 233) 

 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 (rliy nchoccele) 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. 



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FIG. 228. Diagram of the organs of a 

 Nemertine, from below, a. anus ; br. 

 brain ; div. cpeca ; lonci. ne. longitudinal ' 

 nerve-cords ; m. mouth ; n. nephridia ; 

 o-v. ovaries ; pr. proboscis. (After 

 Hubrecht.) 



