408 On the Anatomy and Histology of Nemer tines. 



Annelids— though in tliem only, and not in the Turbellaria, 

 would it be possible to discover them. I will merely draw 

 attention to the fact that with the appearance of this second 

 pair we find that a lateral line appears in the Nemertines as 

 the bearer of sense-organs, precisely as we find it in the 

 Annelids. 



The genital products are either formed directly in the 

 ])arenchyma, in which case a membrane forms round them, 

 constituting a sac, or else they arise in the walls of sacs which 

 alternate with the intestinal cajca. Before maturity is reached 

 a duct is formed, one from each sac. In the non-raetame- 

 rized forms the first of these methods appears to prevail 

 ( Carinella) ^ in the metamerized forms the latter (Cerebratulus, 

 I)repanophorus) . Moreover in these forms, as in Prosadeno- 

 porus, Geonemertes^ and many others, several genital sacs are 

 situated between a single pair of intestinal cceca, and we con- 

 sequently find several genital pores in one metamere. 

 Nemertines are not all of separate sexes : the terrestrial and 

 allied forms, e. g. the Prosadenoporids, are hermaphrodite. 

 Hermaphrodite forms are also found among the Tetra- 

 stemmids, which are closely allied to the Prosadenoporids. 

 Prosorhochmus and Monopora are stated to be viviparous. 



The extraordinarily complicated genital organs of the 

 Turbellaria exclude any comparison with those of Nemer- 

 tines. 



But even as regards the genital organs of the Polychaite 

 Annelids, it is only in their simplicity that those of the 

 Nemertines agree. 



Shortly stated, the conclusion we deduce from the con- 

 siderations which we have discussed in the above pages 

 amounts to this: — That in many respects the organization of 

 Nemertines exhibits an affinity with that of the Turbellaria, 

 but that on the whole this is put into the shade by the general 

 Annelid-like structure of the animals which we have been 

 considering. 



If we merely observe the living fiat Nemertine crawling 

 in its mucus, and compare it with a PolycliKte or an Oligo- 

 chajte, the metamerism of which is exhibited externally by 

 means of rings and the arrangement of bundles of setw, we 

 find but little dilHculty in persuading ourselves to follow our 

 predecessors in the field of natural history and in agreeing 

 Avith the place they assigned to these worms in their classifi- 

 cations — so long, that is, as we ai-e compelled to work with 

 the same appliances as they had. To-day, however, when 

 methods and microscopy have overcome untold dilHeulties 



