GENERAL ACCOUNT OF NEMERTEA. 



187 



d.n 



with the exterior through an open slit or groove, and communicating 

 internally either with the brain itself or with adjacent nervous 

 tissue. In those cases in which the development has been studied, 

 these so-called lateral organs arise from epiblastic insinkings and 

 cesophageal outgrowths. In the most primitive genus, Carine/la, they 

 are absent, except in one species. It has been suggested that they 

 conduce to the respiration of the brain, which is rich in haemoglobin, 

 and they have even been compared with gill-slits. In some forms 

 the groove through \vhich they open to the exterior is rhythmically 

 contractile. It has also been suggested that they are sensory. Apart 

 from these organs, Nemertines are very sensitive, and in many this is 

 associated w 7 ith a superficial nerve plexus. Tactile papilla? and patches 

 are often present : eyes and eyespots are general ; and in some there 

 are otocyst-sacs. Apart from the cephalic slits, the head also bears 

 sensory pits and grooves and terminal sensory spots. In some there is 

 a pair of lateral sense organs 

 in the (anterior) nephridial 

 region. The mouth is ven- 

 tral, and leads into a plaited 

 glandular fore-gut or oeso- 

 phagus, which is followed 

 by a straight, ciliated mid- 

 gut (stomach and intestine), 

 usually with regularly ar- 

 ranged lateral cceca. Between 

 the cceca run transverse muscle 

 partitions. The anus is in 

 most cases terminal. In a 

 cavity along the dorsal 

 median line there lies the 

 remarkable proboscis. It is 

 protruded and retracted 

 through an opening above, 

 or, in a few cases, within the 

 'mouth. It arises as an in- 

 vagination from in front, and 

 is a muscular, very richly 



innervated tube lined with glandular epithelium, sometimes protruded 

 with such force that it separates 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 dra\vn 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 

 containing a fluid with corpuscles (Fig. 89). 



FIG. 91. Transverse section of a simple 

 Nemertean (Carinella).\{\.ei Burger. 



</.., Dorsal nerve ; p.c., proboscis cavity; g., 

 gut; C.HI., circular muscles; /.;;/., longi- 

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

 diagonal muscles ; l.r., lateral blood vessel. 



