External Characters of Arenicola 



33 



EXTERNAL CHARACTERS. All the species of Arenicola are elongate, 

 cylindrical worms, which live, when adult, in sand or mud, or in 

 coarse gravel. Some of the species, especially A. marina, loveni and 

 cmtoto, may attain a length of 400 mm. ; one specimen of A. cribtata 

 was as much as 515 mm. long and 75 mm. in girth. 



Examples of this genus have the morphological components 

 found in other Polychaeta, namely, prostomium, peristomium, a 

 number of body-segments, and a terminal segment or pygidiurn, but 

 the last named has generally been lost in the adult. A certain 

 number, or all, of the body-segments (except the first) bear parapodia, 

 but the latter and the prostomium are considerably modified and 

 reduced, as compared with the corresponding structures of more 

 active, mobile Polychaeta, such as Nereis (PL XII, Figs. 36, 

 37, 38). 



In the caudate species of Arenicola the prostomium (Figs. 1, 2) is 

 a small trilobate structure, 1 which, even in large specimens, seldom 



N.GR. 



PR. 



-PER. 



> A.B.S. 



CH.SEG! 



Fig. 1. Arenicola loveni. Anterior end, dorsal as- 

 pect. PR. Prostomium ; PER. Peristomium ; N.GR. 

 Nuchal groove ; A.B.S. Achaetous body-segment ; 

 CH.SEG. i First chaetiferous segment. 



X 6 



Fig. 2. A, pusilla. Anterior end, dorsal 

 aspect ; showing the large and folded 

 lateral lobes of the'prostomium. 



exceeds 2 mm. in diameter, and, since it is liable to be retracted 

 within the crescentic nuchal groove situated immediately behind it, 

 is often seen with difficulty, and then only in part. The prostomium 



1 Fabricius (Fauna Groenl. (1780), p. 284), in describing Lumbricus papil- 

 losus ( = A. marina), mentioned the presence of a short, foliate, trifid " rostrum," 

 and Savigny drew attention to this small, trilobed " caroncule," but its nature 

 wa*fe not understood thoroughly until Dr. Levinsen (1883) pointed out its 

 homology with the head-lobe (prostomium) of Scalibregma. 



I) 



