194 SYLLID^. 



The Si/llis prolifera is "more studious to divide tlian to unite." 

 At a certain stage of its existence the body of the worm becomes 

 strangulated in the middle, and divides into two after the first rings 

 of the posterior segment have become so modified as to constitute a 

 head. Midler has figured an individual in this condition. But M. 

 de Quatrefages has made the additional and very singular discovery, 

 that the two individuals are endowed with very different functions. 

 The one that was the anterior half of the parent remains unchanged 

 in its natm'e, and probably soon refurnishes itself with a tail; but 

 the second, formed at the expense of the tail, is destined only to 

 multiply the species ! It becomes a uterine mother, and lays aside 

 other duties. The alimentary canal tends to become atrophied, and 

 the animal lives upon itself ; but the generative organs develope, and 

 produce the ova or spermatozoa which are to continue the species*. 



Obs, Of a yellowish-brown colour. The antennae are ciliated, 

 and so twisted with the tentacular cirri that it is often difl^icult to 

 ascertain their numbers and position. 



Plate XV. A. Fig. 3. Syllis jwolifera, of the natural size. 4. The same 

 magnified ; 4 h. the head and anterior segments ; 4 s. the middle seg- 

 ments ; 4 ?. the posterior extremity and styles. 



4. S.? monoceros, head entire in front, furnished with an occipital 

 antenna twice as long as the frontal ; eyes four, quadrangular ; dor- 

 sal cirri about twice as long as the diameter of the body, filiform, 

 jointed. Length 8'" ; breadth about 1", 



Nereis monoceros, Dalyell, Pow. Great, ii. 157. pi. 22. f. 9-13. 



Hab. Coast of Scotland : rare. 



Besc. "Body nearly linear, subdivided into about 70 segments. 

 A long antennular organ, somewhat of a moniliform appearance, 

 issues from the centre of the anterior extremity, and the posterior 

 extremity terminates in a long fork of two similar moniliform organs. 

 Four red eyes, set in a quadrangle, occupy the upper surface of the 

 head, from amidst which the single antennular organ distinctly 

 originates. The site of a large proboscis is denoted by a white part 

 near the anterior extremity. Three kinds of appendages belong to 

 each segment ; a large pencil consisting of a larger pencil of several 

 very transparent, dehcate, bristly hairs ; also a shorter pencU, together 

 with a fleshy spinous prolongation. This animal constructs a slight 

 tube on the side of its vessel." — Dalyell. 



I know two or three other British species of this genus, but I 

 have not been able to describe them, from the ready facility with 

 which they break up into fragments and cast oif their mutdated ap- 

 pendages. The small species are beautifully phosphorescent. One of 

 them Mr. Garner figures under the name oi Nereis phosphor escens\. 



* Ann. & Mag. Nat. Hist. xiii. 235; Rav Soc. Rep. Zool. 1847, 504. 

 t Trans. Zool. Soc. vol. ii. pt. ii. 99. pi. '20. 



