192 THE NEMERTINI 



pairs of cells, each of which gives rise to a " mesoblastic streak," destined 

 to split into a somatic and splanchnic coat, with a small coelom between, 

 is a further support for this view. 



Reproduction and Regeneration. The best known ontogeny is 

 that of the Pilidium (see above) of the Heteronemertines, and of 

 the larva of Desor, which occurs in the life-history of Linens 

 gesserensis (1, 19). 



Internal fertilisation has been definitely observed by Dendy 

 in Geon. australiensis (10). It occurs also in the viviparous forms, 

 such as Prosorhocmus claparedii, etc. 



The Metanemertini have been most studied ; they undergo no 

 metamorphosis, their development is direct, and has recently been 

 studied in several genera by Lebedinsky (26). Nothing is known 

 about the history in Protonemertini, nor Eupolia, and very little 

 in Mesonemertines, but so far as we know it is direct (9). 

 But in addition to this sexual generation, a kind of asexual 

 reproduction exists in the larger species, which, as is well known, 

 break in pieces when attacked, or possibly automatically. Dalyell 

 was the first to recognise that Linens was -able to regenerate a 

 new posterior end. M'Intosh found that the hinder pieces could 

 reproduce a new "head" and proboscis as well as a new tail. 

 Brown (4) has recently made a study of the same species. Burger 

 has investigated the regeneration of the proboscis in Drepanophorus, 

 and Benham (2) has traced some of the histological conditions of 

 a " fragmenting " Carinella, and has suggested that fragmentation 

 may take place, apparently, without any stimulus other than the 

 ripening of the gonads, which are present in this case, only in the 

 " segmented " hinder region of the worm. Hubrecht saw in this 

 power of fragmentation a precursor of metamerism, and there can 

 be little doubt but that this power of separating into pieces, even 

 automatically, is of considerable value to the worm, for the par.t, 

 overburdened with genital products, must be less able to help in 

 the movement of the worm, and thus hinder its escape from 

 enemies, or search for food. If each piece had the power of repro- 

 ducing a head, after expulsion of the genital cells, there would be 

 a close analogy with the asexual reproduction of Syllids and other 

 Annelids. 



LITERATURE OF THE NEMERTINI. 



1. Barrois. (Embryology of Lineus.) Ann. Sci. Nat. (6), vi. 1877. 



2. Benham. (Fission.) Quart. Journ. Micr. Sci. xxxix. 1896, p. 19. 



3. Braun. (Parasitic Forms.) Centralbl. Bakt. Parasitenk. iii. 1888, pp. 16, 



56. 



4. Brown. (Fragmentation.) Proc. Roy. Soc. Ixi. 1897, p. 28. 



5. Burger. (Anatomy and Histology.) Zeit. f. Wiss. Zool. 1. 1890, p. 1. 



