172 



UNSEGMENTED "WORMS: 



with a superficial nerve plexus. Tactile papillae and patches 

 are often present ; eyes and eyespots are general ; arid in 

 some there are otocyst-sacs. Apart from the cephalic slits, 

 the head also bears sensory grooves and terminal sensory 

 spots. The mouth is ventral, and leads into a plaited 

 glandular fore-gut or oesophagus, which is followed by a 

 straight, ciliated mid-gut or intestine, with regularly arranged 

 lateral caeca. Between the caeca run transverse muscle 

 partitions. The anus is in most cases terminal. In a 

 cavity along the dorsal median line there lies the remark- 

 able proboscis. It is protruded and retracted through an 

 opening above, or, in a few cases, within the mouth. It 



arises as an invagination 

 from in front, and is a 

 muscular, very richly 

 innervated tube, some- 

 times protruded with 

 such force that it sepa- 

 rates from the body, and 

 then often retains its 

 vitality for a long time, 

 as if it were itself a 

 worm. It has been 

 compared in its retracted 

 state to a glove -finger 

 drawn in by two threads 

 attached to its tip, the 

 threads being retractor 

 muscles. But in front 

 of the attachment of the retractor muscles there is a non- 

 eversible glandular region which secretes an irritant fluid. 

 In many cases there are stilets at the tip of the eversible 

 portion, and if these be absent, there are stinging cells or 

 adhesive papillae. There is a hint of a similar structure 

 in some Turbellarians, and the organ may be interpreted 

 as one which was originally tactile, but which has become 

 secondarily aggressive. It is protruded by the muscular 

 contraction of the walls of the proboscis sheath, which 

 forms a closed cavity surrounding the proboscis, and con- 

 taining a fluid with corpuscles (Fig. 75). 



In the majority there are three longitudinal blood vessels 



Fig. 77. — Transverse section of a simple 

 Nemertean [Carinella). — After Burger. 



d.ti., Dorsal nerve ; p.c. t proboscis cavity; g., 

 gut; cm., circular muscles; l.m., longi- 

 tudinal muscles ; d.v.m., dorso-ventral or 

 diagonal muscles ; l.z>. t lateral blood vessel. 



