BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 371 



proboscis, and other organs, before they leave the body of the parent, and on being 

 set free have the same general form as the latter. The larger examples are often 

 doubled within the body of the parent, and apparently invested by the stretched 

 covering of the ovisac, or in large cavities produced by the coalescing of many 

 ovisacs; at any rate, it is clear that to describe them (as Prof. Keferstein and 

 M. Claparede* have done) as simply within the body-cavity of the worm, is want- 

 ing in structural accuracy. It is certainly a curious sight to see these large young 

 animals moving within the body of the adult, apparently without causing the 

 latter any inconvenience. Such, then, appears to be a further stage of the type of 

 development seen in certain species (e.g., Polia involuta, Van Beneden), in which, 

 after deposition of the majority, a few ova are left in the body of the parent for 

 subsequent evolution. It remains, however, to be proved whether all the ova in 

 Prosorhochmus are so developed (in which case they must be very few), or whether 

 part are deposited at one or different periods or stages, and the rest evolved in 

 the body of the parent. By the examination of this species, I have been enabled 

 to confirm many of the excellent observations of Prof. Keferstein and M. 

 Claparede ; but, on the other hand, the determination of the actual position of 

 the mouth in the same animal shows that it does not deviate from the typical 

 Ommatopleans, and that the organ is situated not behind the ganglia, as asserted, 

 but, like the others previously described, quite in front of the commissures. 

 The mouth, moreover, is the most distinct of any I have examined. 



It appears to me that such viviparous species do not form a group sui generis. 

 but are connected by insensible gradations with the true oviparous forms. Doubt- 

 less, in the majority, some of the ova only are retained in the ovisacs, impreg- 

 nated by the ubiquitous spermatozoa through the genital pores, developed in the 

 sacs, and space afforded for the growth of the young animals by the stretching 

 or rupturing of the membranous walls of the latter. It is a very interesting fact 

 in connection with this subject, that Prof. Keferstein f has lately discovered a 

 Hermaphrodite Nemertean (Borlasia hermaphroditica) at St Malo, in which the 

 anterior sacs were found full of mature spermatozoa, and the. posterior distended 

 with developing ova. This can only be explained in one of two ways — either 

 that the species is truly a hermaphrodite one, or that the spermatozoa are passed 

 from the body of a male (in apposition) into certain sacs of the female through 

 the genital pores, there to remain until the other contents of the female generative 

 organs are evacuated. 



BORLASIA. 



Cuticular Tissues. — The skin in this group, for which Borlasia olivacea may 

 be taken as the type, is allied in structure to that of Ommatoplea, though 

 in the living animal its condition is frequently rendered obscure by the much 



* Beobachtungen tiber Anal, &c, p. 23. 



f Ann. Nat. Hist., 4th ser. vol. i. 1868, p. 229 ; and Archiv fur Naturges. 1868. 



