MUSEUM OF COMPARATIVE ZOOLOGY. 93 



and a liniug to the body wall and alimentary tract, to a single thiu cell 

 layer lying next to the ectoderm, or to meseuchymatous cells extending 

 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 Alcyonidium 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- 

 cies, and in some cases possess only the rudiment of the internal sac. 



If we were to imagine still another term at the degraded end of the 

 series, it would be a form in which the four inner-layer cells that arise 

 by ingression 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 

 entirely absent. It is just these conditions which are fulfilled by the 

 Phylactolrematous larva. 



Of all these changes, the loss of the entoderm is the most strikinsr. 

 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 taken 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 Cyphonautes 

 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 Fran^aise pour I'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 



