290 ZOOLOGY SECT, v 



ORDER 2. MESONEMERTINI. 



Dimyarian Nemertines having the lateral nerve-cords withdrawn 

 within the musculature of the body-wall. The mouth is situated 

 behind the brain. There are no stylets. 



ORDER 3. METANEMERTINI. 



Dimyarian Nemertines, in which the lateral nerve-cords lie 

 inside the muscular layers in the parenchyma. The mouth is 

 situated in front of the brain. The proboscis is provided with 

 stylets. A ventral caecum is present. 



ORDER 4. HETERONEMERTINI. 



Trimyarian Nemertines, in which the lateral nerve-cords are in 

 the muscle-layers, between the outer longitudinal and the circular 

 layers. The mouth is situated behind the brain. The proboscis 

 has no stylet. 



The Nemerteans are almost exclusively marine ; and the greater 

 number live between tide-marks or at moderate depths ; a few 

 have been obtained from considerable depths. The comparatively 

 small number of terrestrial and fresh-water forms are all 

 Metancmertini. The Nemerteans progress for the most part by 

 slow crawling movements, leaving a track of slime behind them. 

 Some burrow freely in mud or sand, the proboscis being made use 

 of to help in the process. Some are able to swim by means of 

 undulating movements of the body. Nearly all are carnivorous, 

 and either capture living prey in the shape of small invertebrates 

 of various kinds, or feed on dead fragments. The chief function 

 of the proboscis is the capture of living prey, around which it 

 becomes coiled and then draws the prey towards the mouth. One 

 Nemertean lives in the interior of a Crustacean, and is probably a 

 true parasite. Others, live, apparently as commensals or mess- 

 mates, in the pharynx or atrial cavity of Ascidians, or within the 

 mantle cavity of bivalve Mollusca. 



A striking feature of the Nemerteans is the readiness with 

 which, on being irritated by handling or by the action of some 

 chemical agent, they break up transversely into fragments. This 

 takes place most freely when the body is highly charged with 

 sexual products, but is by no means confined to that condition. 

 The process probably takes place spontaneously under certain 

 circumstances. The broken-off fragments may remain alive for a 

 considerable time, and under suitable conditions regeneration of 

 the lost parts is readily effected, so that it is possible to look upon 

 the entire process as a form of asexual reproduction. 



