INTRODUCTION EXTERNAL CHARACTERS 



IOI 



very 



great force. This thread (Fig. 50, p) is the proboscis, a 

 important and characteristic organ in Nemertines. 



Most Nemertines are marine ; they are 

 mostly indifferent to climate and to the 

 nature of the soil on which they live. 



A few forms live on land (e.g. Tetra- 

 stemma agricola, 1 Geonemertes palaensis? 

 and G. chalicopliora*) or in fresh water 

 (e.g. Tetrastemma aquarum dulcium* and 

 T. lacustre 5 ) in various parts of the globe. 

 There are also parasitic forms ; the best 

 known of which is Malacobdella. 6 A 

 pelagic form, Pelagonemertes? has been 

 described by Moseley. 



External Characters. A typical 

 Nemertine possesses an elongated worm- 

 like body (Fig. 49), which is usually 

 thrown into numerous close coils (Fig. 

 48). In section it may be either round 

 or more or less flattened, with the lateral 

 edges in some cases quite thin and almost 

 iin-like. One or two broad, flattened, and leaf-shaped forms are 

 known, but such a condition is exceptional, and the forms in 

 which it occurs have probably assumed it owing to the adoption 

 of special modes of life. 



In the ordinary forms the posterior end of the body is pointed 

 either bluntly or sharply. The head is somewhat broader than 

 the rest of the body, and often assumes a spatulate form. Eyes 

 (Fig. 51, e) are usually present either in one or several pairs, or 

 in symmetrically-arranged groups on each side of the head. The 

 mouth (Fig. 58, m) is situated near the front end of the body 

 on the ventral surface, and is usually rendered conspicuous by 

 being surrounded by thick tumid lips. It varies in form from 

 being slit-like to elliptical. At the anterior end of the body a 



1 R. von Willemoes-Suhm, Ann. Nat. Hist. ser. iv. xiii. 1874, p. 409. 



2 Semper, Zeitschr. wiss. Zool. Bd. xiii. 1863, p. 558. 



3 L. von Graff, Morphol. Jahrb. Bd. v. 1879, p. 430. 



4 W. A. Silliman, Zeitschr. wiss. Zool. Bd. xli. 1885, p. 48. 



5 du Plessis, Zool. Anz. vol. xv. 1892, p. 64. 



6 J. von Kennel, Arb. Inst. Wiirzburg, Bd. iv. 1877-78, p. 305. 



7 H. N. Moseley, Ann. Nat. Hist. ser. iv. vol. xv. 1875, p. 165. 



Fig. 50. Side view of head 

 of Cerebratulus {Mi- 

 crura) tristis Hubr., show- 

 ing the everted proboscis. 

 Naples. x 2. Drawn 

 from a spirit specimen, c.s, 

 Cephalic slit ; m, mouth ; 

 p, proboscis. 



