The Sexual Phases of Myzostoma. 
283 
ctenidae, E. Meyee; Glyceridae, Eisig '87; Naidae, Semper '76; 
Vejdovsky '84). The facts which seem to me to favor this homology 
are the following: 
1) The sacs of the Myzostomidae are metameric and lie lateral 
or dorsal to their respective parapodia like the Seitenorgane of 
Capitellidae. 
2) The segmental sacs like the Seitenorgane are eversible and 
provided with a special set of retractor mnscles (Eisig’s Haarfeld- 
retractor) . 
3) The general structiire of the segmental sacs is not unlike 
that of the »Seitenorgane« as described and fignred 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 motile, 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 »Körnerzellen«. It may also be remarked that Eisig found 
yellow granulös in the Seitenorgane of Capitellids, and we have 
seen that yellow granulös also occur in the segmental sacs of M. 
platypus. 
The all-important point in the homology, viz. the presence of 
a nerve supplying the large cells of the segmental sac, I have 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 segmental sacs might be sensory, and so the oppor- 
tunity of employing methylene blue and GolgTs 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 Ärthropoda. 
Certainly in the embryo Limulus the organs designated as -sensory 
by Patten ('89, '90) and Kingsley ('90, '92, '93) have a striking 
superficial resemblance to the segmental sacs of Myzostoma. The 
exact nature of these organs is extrem ely doubtful, and the difficulty 
of deciding between the same alternatives which have just been 
discussed, is apparent in Kingsley’s paper. After speaking of these 
Organs in 1890 as »plainly sensory«, they subsequently become »more 
