iv PROBOSCIS OF THE AGANTHOCEPHALA 199 



and which, at the external aperture of the latter, passes ^into the outer 

 body epithelium. 



At the blind posterior end of the proboscis a strand of muscle 

 fibres is inserted, the retractor (rm) of the proboscis, which runs 

 freely inside the proboscis sheath to its posterior end, breaking through 

 its walls to lose itself in the dorsal longitudinal musculature. The 

 proboscis can be evaginated from the proboscis sheath. This occurs 

 principally by a contraction of the muscular walls of the proboscis 

 sheath. In an evaginated condition the proboscis projects from the 

 anterior end of the body as a long tube, while the rhynchodseum 

 remains in its position. The inner wall then lies outside, the outer 

 wall inside ; the blind posterior end is at the extreme point of the 

 protruded proboscis, and its epithelium then represents still more clearly 

 than before a simple continuation of the outer body epithelium. The 

 anterior part of the extended retractor then lies in the central cavity 

 of the proboscis into which the fluid of the proboscis sheath penetrates. 

 By contraction of the retractor the proboscis is again invaginated. 



At the blind end of the invaginated proboscis there is found in the Hoplonemer- 

 tina, a stylet (st] projecting into the proboscidal cavity, and at its side accessory stylets, 

 mostly small and in the course of formation. These stylets, when the proboscis is 

 fully protruded, come to lie at its foremost end and project freely. Further, the duct 

 of a sac-like gland (poison gland) (gd) to whose posterior end the retractor is 

 attached, is also often found at the end of the proboscis. In the unarmed Nemertina 

 numerous rod-shaped bodies or stinging capsules lie in the epithelium of the pro- 

 boscis. Only in Amphiporus, Malacobdella, and Geonemertes palaensis the probos- 

 cidal aperture opens into the oesophagus from above, so that the proboscis is protruded 

 through the mouth. Few observations have been made as to the function of the 

 Neinertian proboscis, which when irritated is generally so energetically projected that 

 it tears itself off at its edge of insertion in the rhynchod?eum. It probably serves as 

 a weapon both for defence and offence. 



We have already in the Platodes become acquainted with organs similar to the 

 proboscidal apparatus of the Nemertina. The proboscis of the Proboscidea (p. 150) 

 among the Wiabdoccela (Turbellaria), representing an invagination of the most 

 anterior body wall which has become permanent, may possibly be homologous with 

 the Nemertian proboscis. There is a further extraordinary correspondence in general 

 arrangement between the proboscidal apparatus of the Nemertina and each of the 

 4 proboscidal apparati of the TetrarJiynchidce among the Cestoda. 



The proboscidal apparatus of the Acanthocepliala (Fig. 172, p. 

 258) consists of the following principal parts: (1) the proboscis, (2) 

 the proboscis sheath, (3) the proboscis retractor, (4) the retractors of 

 the proboscis sheath, (5) the retinacula. 



The proboscis in its evaginated condition represents the cylindri- 

 cal or conical anterior end of the body, narrowed and outwardly armed 

 with numerous hooks or stylets. The proboscis sheath is a muscular 

 pouch with a double wall, closed on all sides. It is attached at the 

 base of the proboscis to the body wall, and projects thence backwards 

 into the body cavity. It receives the proboscis when the latter is 

 invaginated. By its contraction the proboscis is protruded. The 



