1891.] A Commensal Anneltd. 25 
is, at its greatest development, nothing more than a highly com- 
plicated, self-supporting pod. 
The sporophyte, then, may be considered in its widest sense as 
a result of vegetative reproduction applied to the fertilized egg. 
As the thallus of Marchantia is cut into gemme, so the egg of 
Marchantia may be cut into the cells of the sporophyte. The 
strict analogy of sporophytes.is, I suppose, not questioned. The 
question as to their homology must be studied from the physio- 
logical point of view, as well as through the researches of ana- 
tomists and embryologists. Without either dissenting from or 
endorsing the view of Bowers, the writer has tried only to show 
that emphasis may be laid upon conditions surrounding the 
sexual act as a help toa clearer comprehension of the phenomena 
of alternation. 
A COMMENSAL ANNELID. 
BY E. A. ANDREWS, PH.D. 
ASES in which Annelids are believed to live more or less 
directly dependent upon other Annelids, upon Crustacea, Gas- 
teropods, Echinoderms, and even upon Ccelenterates, are not un- 
‘known, but yet form by far the exception rather than the rule in 
the economy of this group. Of the reported cases some must be 
regarded as mere temporary refuge of Annelids in cavities offered 
by the shells or bodies of other creatures ; some are such mechan- 
ical associations as are presented in the complicated assemblage 
of various Annelid tubes in sponges or molluscan shells, etc. ; 
while yet others—and these are few—are illustrations of true 
commensalism, which may pass over into parasitism, as in the 
Oliognathus living in Bonellia, the Hzematocleptes living within 
another Annelid, or the well-known Alciope living inside 
Ctenophores. 
In 1885 Dr. Brooks called my attention to a very interesting 
case of commensalism involving an Annelid found at Beaufort, 
N. C., and which is complicated by the fact that three diverse 
