MUSEUM OF COMPARATIVE ZOÖLOGY. 91 
layer in the different Ectoproct larvæ that we must look for an explana- 
tion of the layer in the specific case of the Phylactolemata. 
For the purposes of this study, it is desirable to begin with species in 
which there has been a minimum amount of degeneration. Such are 
Membranipora (Cyphonautes), Alcyonidium, and Flustrella, to which 
we must now turn our attention. 
The studies of Repiachoff on Membranipora lead up to a stage in which 
the entoderm lies as a solid mass inside the ectoderm, and is separated 
from it at all points. Neither the origin of the mesoderm nor the forma- 
tion of stomodeum or proctodeum was observed at this time. As for 
the fully formed Cyphonautes, it is certain, as I can confirm from personal 
observation, that there is a well developed functional alimentary tract, 
and that it is provided with a well developed muscular system, including 
cross-striped muscle fibres. There is, therefore, every reason for believing 
that typical entoderm and mesoderm have been formed in it, 
In Alcyonidium (polyoum), Harmer (87, p. 445) has shown that after 
gastrulation a great mass of cells occupies the former blastocel. This, 
in the author’s opinion, represents entoderm and mesoderm. The young 
larva possesses a mouth, ‚an esophagus, and a large stomach, but never 
an anus. No evidence is presented that the oral pole corresponds with 
the pole of ingression. 
Flustrella, which is nearly related to the last species, possesses in 
its young larval stages a pocket, which Prouho (’90, pp. 424-426) has 
shown to represent the anterior part of the alimentary tract, directly 
comparable with that of Aleyonidium polyoum, but less developed. Mus 
cle fibres and an epithelial lining of the entoderm and ectoderm exist to 
indicate the presence of mesodermal tissue. 
These three genera, Membranipora, Aleyonidium, and Flustrella, are 
the only Ectoprocta in whose larvae the presence of an alimentary tract 
has as yet been demonstrated. 
In Bugula, a very careful study of which was made by Vigelius (86 
and ’88), one finds after gastrulation and cell multiplication a mass of 
cells filling the whole interior of the larval body, at first appearing as 
an epithelium surrounding a central space, but later without arrangement 
and often showing signs of degenerescence. No definite separate meso- 
derm could be found, and at no time was any trace of'an alimentary tract 
to be seen. Vigelius calls the mass derived from the four entodermal 
cells Füllgewebe, and he believes it to correspond morphologically to 
both “hypoblast and mesoblast.” Tt is to be noted, however, as a 
point of considerable importance, that in his figures of the metamorphos- 
