284 
W. M. Wheeler 
probably glandulär« ^ Both Patten and Kingsley agree, however, 
that one pair of these organs (viz. the fourth) gives rise to the so- 
called »dorsal organs« of Limulus, This fact is of interest, for if 
these organs in Limulus be really embryonic lateral line organs, we 
may yet be able to trace out the phylogenetic history of such enig- 
matica! structures as the paired »dorsal organs« of Mysis, the paired 
lobulated outgrowths on either side of the head in the embryo 
AselluSj etc. 
The fact that in Myzostoma there are five pairs of parapodia, 
but only four pairs of segmental sacs, naturally leads to the question 
as to what has become of the missing pair of sacs. The answer 
to this question I b elieve we need not go far to seek: the third 
pair of the original five pairs of sacs has been converted 
into the so-called penes. These are more or less prominent 
papillae lateral to and near the bases of the third pair of parapodia. 
Each papilla is perforated by a ductus ejaculatorius which widens 
proximally into a vesicula seminalis. The latter receives the mature 
spermatozoa from the vasa deferentia and these in turn from the 
ramifying testicular follicles^. 
Bizarre as the development of a male reproductive organ from 
a lateral line organ may appear at first sight, I am nevertheless 
unable to see any great difficulty in such a change of function. It 
is in fact easy to see how the bottom of an eversible sac might acquire 
au opening into the body-cavity under the pressure of a great ac- 
cumulation of spermatozoa; the sac would then become reduced to 
a mere conduit. l'he histological difficulties are not insuperable, 
for the large infolded ectodermal cells which form the walls of the 
ductus ejaculatorius may be homologized with the large succulent 
cell forming the walls of the segruental sacs. Another fact which 
Supports the view here advocated is the correlative Variation of the 
segmental sacs and penes in different species of Myzostoma. In 
■platypus e, g., where the sacs are extremely large, the penes also 
attain to extraordinary dimensions, whereas both sets of organs become 
1 Cf. also Kingsley's remarks at pag. 538, '90; pag. 51 and 52, '92, and 
pag. 222, '93. 
2 It is hardly correct to call the papillae penes, since the Myzostomidae 
do not copulate. Even in M. platypus ^ where these organs are enormously de- 
veloped, they are apparently only intended to be thrust through the orifice of 
the gali to disseminate the spermatozoa in the sea-water. 
