82 ANNELIDES. 



Lost portions are restored with facility. A few secrete a luminous 

 fluid, but an organ appropriated to the secretion has not been demon- 

 strated. The skin is very generally margaritaceous or iridescent ; 

 and this is occasionally the case also with the bristles. 



1 . General Form. — The Annelides have an elongated worm-like 

 figure, which, in some genera, inclines more or less to an oblong or 



No. VI. 



a. Animal. b. Head. e. Jaws. d. Rings. 



oval. TheNereides oifer examples of the vermiform species (No. VI.); 

 the Polynoc, and, more especially, the Aphrodites, may be instanced 

 as examples of the latter. The length is often considerable. On our 

 shores species are to be found nearly two feet in length, and as 

 thick as the barrel of a large quill ; but in equatorial seas some 

 attain the length of five feet with a diameter of thirteen lines. 



2. Body. — The body is composed of narrow segments or rings 

 (No. VI. fig. d), not calcareous nor even corneous as they are in the 

 great majority of Crustacea and insects, but membranous, and merely 

 separated by a fold of still thinner membrane, such as we observe in 

 many larvae and caterpillars, so that it is occasionally difficult to 

 mark their exact limits. The number of the rings is in general very 

 considerable, and proportionate to the length of the body, for the 

 growth of this in length depends much more on the production of 

 new segments than on the development of any one in particular. 

 There are great diff'erences in the number of rings necessary to com- 

 plete maturity. In some Polynoc there are not more than from 

 20 to 30 rings ; in Phyllodoce lamellata not less than 500. In 

 the species which have few rings, as in Aphrodite and Polynoe, 

 the number appears to be specifically limited, and the same in all 

 the individuals ; but in the Nereides and others nothing is more 

 variable, and less to be relied upon as a discriminative character. 

 This variety depends on age, on circumstances more or less favourable 

 to growth, and on the eff'ects of injuries. 



3. Head and Appendages. — The first segment forms a head 

 more or less distinct (No. VII. fig. I). On the upper or dorsal surface 

 of this head there are usually one or two pairs of black specks, 

 believed to be eyes* (fig. 1, a). The head bears also usually a 



* Blainville doubted whether they were eyes (Diet, des Sc. Nat. xlvii. p. 210). 

 But Midler believes them to be really visual organs, for they have nerves from 

 the brain, the form of eyes, and are provided with a black pigment. He admits, 



