The Sexual Phases of Myzostoma. 
283 
ctenidae, E. Meyer; Glyceridae, Eisig '87; Naidae, Semper '76; 
Vejdovsky '84). The facts which seem to me to favor this homologj 
are the foUowing: 
1) The sacs of the Myzostomidae are metamerie and lie lateral 
or dorsal to their respective parapodia like the Seitenorgane of 
Capitellidae. 
2) The segmentai sacs like the Seitenorgane are eversible and 
provided with a special set of retractor muscles (Eisig's Haarfeld- 
retractor) . 
3) The general structure of the segmentai sacs is not unlike 
that of the » Seitenorgane « as described and figured by Eisig. In 
both cases the well-developed cuticle covering the. surface of the 
organ is provided with cilia which are easily destroyed by reagents 
and are very probably not metile, but sensory in function. The large 
gland-like cells in the sacs may be in reality ganglionic — at least 
they closely resemble the cells which Nansen regards as parapo- 
dial ganglia. If this is the case, they would probably correspond 
to Eisig's »Körnerzellencc It may also be remarked that Eisig found 
yellow granules in the Seitenorgane of Capitellids, and we bave 
seen that yellow granules also occur in the segmentai sacs of M. 
platypus. 
The all-important point in the homology, viz. the presence of 
a nerve supplying the large cells of the segmentai sac, I bave not 
yet been able to establish; but it should be remembered that it is 
no easy task to detect the nerve which supplies the lateral line organ 
of the Chaetopods. Unfortunately it did not occur to me while at 
Naples that the segmentai sacs might be sensory, and so the oppor- 
tunity of employing methylene blue and Golgi's method on fresh 
material was allowed to escape. 
Eisig has extended the homology of the «Seitenorgane« of Chae- 
topods to the lateral line organs of Vertebrates. It seems to me 
that it may be extended also to certain organs in the Arthropoda. 
Certainly in the embryo Limulus the organs designated as [sensory 
by Patten ('89, '90) and Kingsley ('90, '92, '93) bave a striking 
superficial resemblance to the segmentai sacs of Myzostoma. The 
exact nature of these organs is extrem ely doubtful, and the diffìculty 
of deci ding between the same alternatives which bave just been 
discussed, is apparent in Kingsley's paper. After speaking of these 
organs in 1890 as «plainly sensory(f, they subsequently become »more 
