MUSEUM OF COMPARATIVE ZOOLOGY. 93 
all and alimentary tract, to a single thin cell 
and a lining to the body w 
or to mesenchymatous cells extending 
layer lying next to the ectoderm, 
through the coelom. 
This same series may be said, also, to be one in which there is a 
gradual decline in the complexity of larval organs. These find their 
maximum development in the bivalve Cyphonautes and Flustrella, and 
the complicated and beautiful Aleyonidium larva. They find their mini- 
mum development in the Cyclostomes, whose larvae, instead of a girdle of 
flagella, possess merely an undifferentiated clothing of cilia, are reduced 
to a cylindrical or ellipsoidal form, lack the pyriform organ of other spe- 
udiment of the internal sac. 
cies, and in some cases possess only the r 
he degraded end of the 
If we were to imagine still another term at t 
series, it would be a form in which the four inner-layer cells that arise 
at one pole of the larva should give rise to little or abso- 
lutely no entoderm, in which the mesoderm should come to form an 
inner lining to the ectoderm, and in which the internal sac should be 
It is just these conditions which are fulfilled by the 
by ingression 
entirely absent. 
Phylactolematous larva. 
Of all these changes, the loss of the entoderm is the most striking. 
What can be said in explanation of it? I would suggest this hypoth- 
esis: that the entoderm of the Bryozoan larva has become rudimentary 
through loss of the alimentary function. 
In direct support of this hypothesis I have little experimental evidence 
to offer. One observation, however, which I made last summer, seems 
to favor this conclusion strongly. This is that larval life is of consider- 
able duration in Cyphonautes, which possesses a functional alimentary 
tract, but is very brief in Bugula, in which no alimentary tract arises. 
As is well known, Cyphonautes occurs in enormous numbers in the 
“tow ” at certain seasons of the year, and this is alone evidence of a con- 
siderable length of life. I have t: ken Cyphonautes thus obtained from 
the tow and have kept them for three or four days, at the end of which 
time they died, or had settled to the bottom of the glass vessel to un- 
dergo their metamorphosis. In fact, from several hundred Cyphonantes 
which I collected, not more than half a dozen completed their full meta- 
morphosis, the others apparently succumbing to unfavorable conditions. 
1 Just as the manuscript of this paper is going to the printer, after long delay 
caused by an accident necessitating the re-engraving of the plates, I find that Dr. 
Prouho read last summer (’90), before the Association Francaise pour l’Avancement 
de la Science, a preliminary communication on the development of Cyphonautes. 
This is published in the printed report of the proceedings of that association. The 
