342 PLATYHELMUJTHES. 



ance, forming the first rudiment of the Nemertine body. The four 

 discs so formed fuse together and give rise to a ventral germinal 

 plate, which gradually grows round the alimentary canal of the 

 Pilidium to form the skin of the future Nemertine. The proboscis 

 arises as an invagination of the anterior end of the germinal plate 

 (fig. 278). The young Nernertine subsequently breaks through 

 the larval skin. 



The Nemertines live principally in the sea, under stones in the 

 mud, but the smaller species swim about freely. There are also 

 forms which live on the land, as well as pelagic forms. Certain 

 species form tubes and passages, which are lined with a slimy secre- 

 tion. The food of the larger species principally consists of tubicolous 

 worms, which they extract from their habitations by means of the 

 proboscis. There are, however, parasitic JSTemertines which infest 

 Crustacea or live on the mantle and gills of Mollusca. In this case 

 they are, like the Hirudinea, furnished with a posterior sucker 

 (Malacobdella). The Nemertines are distinguished by their repro- 

 ductive capacity and by their tenacity of life. Mutilated parts are 

 quickly regenerated, and the parts into which certain species readily 

 break are said to have the capacity, under favourable conditions, of 

 developing into new animals. 



1. Sub-order : Enopla. The proboscis is armed with stylets. 

 The short, often funnel-shaped cephalic slits are connected with 

 lateral organs, which correspond to the posterior cerebral lobes of 

 the Anopla. In the brain the upper lobes are slightly elongated 

 posteriorly leaving the ventral lobes, from which the lateral nerves 

 arise, quite free. Development takes place without metamorphosis. 



Fam. Amphiporidae. The ganglia are more rounded, the lateral nerve 

 trunks are placed inside the dermal muscles. The mouth is on the ventral 

 surface near the anterior end of the body, in front of the commissures between 

 the ganglia. The lateral organs are separated from the brain and connected 

 with it by fibres ; they contain a narrow water canal. Amphiporus lactifloreus 

 Johnst. Lives under stones, and is distributed from the North Seas to the 

 Mediterranean, 3 4 in. long. A. spectabilis Quatr. Borlasia sphiidlda Kef., 

 Mediterranean, and Adriatic. Tetrastcmma olscuruvi M. Sch. Viviparous : 

 Baltic. T.agricola Will. Suhm., terrestrial. Nemertes gracilis Johnst. 



2. Sub-order: Anopla. The proboscis is unarmed. The long 

 cephalic slits occupy the whole side, or the anterior part of the head, 

 and lead into the lateral organs, which are direct processes of the 

 upper lobes of the brain. Development frequently by means of 

 ciliated larvae. 



