408 On the Anatomy and Histology of Nemertines. 
Annelids — though in them 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 
parenchyma, 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 caeca. Before maturity is reached 
a duct is formed, one from each sac. In the non-metame- 
rized forms the first of these methods appears to prevail 
( Garinella ), in the metamerized forms the latter ( Cerebratulus , 
Drepanophorus). Moreover in these forms, as in Prosadeno- 
porus y GeonemerteSy and many others, several genital sacs are 
situated between a single pair of intestinal caeca, 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 Polychaete 
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 flat Nemertine crawling 
in its mucus, and compare it with a Polychaete or an Oligo- 
chaete, the metamerism of which is exhibited externally by 
means of rings and the arrangement of bundles of setae, we 
find but little difficulty in persuading ourselves to follow our 
predecessors in the field of natural history and in agreeing 
with the place they assigned to these worms in their classifi- 
cations — so long, that is, as we are compelled to work with 
the same appliances as they had. To-day, however, when 
methods and microscopy have overcome untold difficulties 
