90 BULLETIN OF THE 
surrounded by a circular fold, — the so-called mantle fold, — from which 
it is separated by a circular groove, — the so-called mantle cavity. This 
organ has a similar origin and fate in the two groups, as shown by Barrois. 
The following points of difference, however, must be recognized. 
First, the absence of a definite ciliated ring, cowronne (Barrois), of an 
internal sac, and of a pyriform organ. But, as Barrois (’86, p. 67) has 
shown, these are absent, or at least (Ostroumoff, ’87, pp. 182, 183) little 
developed, in Cyclostomatous Bryozoa. The ciliated ring and pyriform 
organ are doubtless organs connected with a free locomotive larval life, 
which is greatly abbreviated in Phylactolemata. A second difference 
exists in the fact that, while most Gymnolematous larvae possess either, 
rarely, (1) a functional alimentary tract, or (2) a mass of loose tissue 
lying inside of the ectoderm, the Phylactolemata possess (3) a central 
space lined by an epithelium placed next to the ectoderm. However 
great the difference between the first and third conditions mentioned 
above, it is to a large extent bridged over by the widespread existence of 
the second. In some Cyclostomes, moreover, a similar condition to that 
in Phylactolemata seems to exist. Compare Metschnikoff (’82, p. 310, 
Taf. XX. Fig. 62). Lastly, the origin of two primary polypides, instead 
of one, at the aboral pole, upon which Barrois has laid some stress, can- 
not be considered a very strong objection to the homology, because in 
reality the two polypides do not arise at the same time even in Pluma- 
tella, and in Cristatella this difference is still more pronounced. In fact, 
it is not the formation of two polypides which requires explanation, but 
that of a young stock before hatching. 
There remains, therefore, to my mind, no serious objection to regard- 
ing the larve of Phylactolemata and Gymnolemata as having been 
derived from some common ancestral larva, possessing, of course, more 
points of resemblance to the Gymnolematous than to the Phylactolema- 
tous type; and therefore it is perfectly justifiable to interpret the latter 
by aid of the former. 
Admitting the larva to be homologous, we should expect the process 
of gastrulation to be comparable throughout Ectoprocta. As a matter 
of fact, we do find a great similarity in the »arliest stages. Thus, the 
first indication of the inner layer is the ingression of four cells at one 
pole, which by multiplication give rise to a layer of cells lying inside of 
the ectoderm.’ It is to the comparative study of the fate of this inner 
1 This has been shown for Membranipora (Tendra) by Repiachoff (’78, pp. 
416-420); for Alcyonidium polyoum by Harmer (’87, pp. 445, 446); for Bugula 
by Vigelius (’86, p. 519) ; and for Cristatella in the present paper (page 68). 
