Ill 



PL A TODES SENSOR Y ORGANS 



151 



condition. The proboscidal apparatus of such an animal consists of an 

 invagination at the anterior end of the body, on to which the outer 

 epithelium is continued. This invagination can be evaginated and 

 again withdrawn by means of special retractors. The invagination, as 

 well as the retractors which are inserted in it, are surrounded by a sac- 

 like muscular integument, by whose contraction the proboscis is pro- 

 truded. Fig. 108, A, JB, C, represents the proboscis in various stages 

 of protrusion. The whole apparatus shows in every detail a similarity 



FIG. IDS. Proboscis of Macrorhynchus croceus. A, In a protruded condition; B, half protruded; 

 C, withdrawn (after v. Graff), re, Proboscis epithelium, which is a continuation of the body epi- 

 thelium (e); ms, muscular envelope which divides the proboscis from the body parenchyma; m, 

 dermal muscle layer ; ra, muscles for withdrawing the proboscis. 



which cannot be ignored to the 4 proboscides of the Tetrarhynchidce and 

 the proboscis of the Nemertina to be described later. 



In the Trematoda the sensation of touch seems specially localised 

 in the suckers. 



D. Ciliated Pits. 



In certain Bhabdoccelidce, viz. in the Microstomidce, Prorliynchidce, 

 and PlagiostomidcB, there are 2 paired strongly ciliated integumental 

 pits which lie laterally on a level with the brain, and are supplied 

 with a nerve ring by the brain. They have been regarded as olfactory 

 pits. In the Triclada, also, similar strongly ciliated parts of the 

 epithelium of the end of the head have been observed, to which special 

 sensory nerves proceed. In Bipalium, a land Tridad, there are pits 

 supplied with special nerves in large numbers at the anterior margin 

 of the body which is broadened out in the shape of a crescent. Whether 

 the ciliated furrow which in all Polydada runs along the anterior margin 

 of the body in the epithelium of the ventral side belongs to the forma- 

 tion here described cannot yet be decided. 



X. The Body Parenchyma (Retieulum). 



The whole space between the body wall and the gastro-canal system, 

 as far as it is not filled by specific organs, is occupied by a cellular 

 connective tissue, the details of whose structure are very various. This 

 connective tissue, which corresponds with the gelatinous tissue of the 

 higher Cnidaria, is called parenchyma or reticulum. It often becomes 



