370 DR. ALLMAN ON THE ANATOMY AND PHYSIOLOGY OF CORDYLOPHORA. 
the neck and clavate body of the polype, at least as far as the roots of the posterior 
tentaciila. 
It is certain that the cavity of the polypary increases m diameter, up to a certain 
point with the growth of the animal, a fact which we can reconcile with the account 
just given of its formation, only by attributing to it a capacity of distension subse- 
quently to its original deposition. 
2. The Endoderm and its Cavity. 
The endoderm (fig. 3, d, d) constitutes a very distinctly cellular layer ; it is in 
contact throughout with the ectoderm, but is nevertheless distinguishable from it by 
a very decided boundary, and can only be considered as directly continuous with it 
at the mouths of the polypes. It forms the walls of an uninterrupted cavity, which, 
extending through the axis of the entire zoophyte, opens externally at the polype- 
mouths. This cavity may be divided into four distinct regions, namely, the post-hucml 
cavity the stomach, the tentacular canals, and the canal of the ccenosarc, a division 
which, though in some degree arbitrary, will be found very useful in description. 
(a.) PosCbuccal cavity. —The post-buccal cavity (fig. 3/) is situated immediately 
- behind the mouth (fig. 3 e), which is a simple unarmed orifice placed at the extremity 
of a conical projection, into which the body of the polype is continued anteriorly. 
The post-buccal cavity lies in the axis of this projection; its walls are formed of 
elongated cells, with their long diameter perpendicular to the surface, and its interior 
is quite destitute of rugae. It is usually in the form of a narrow tube, and is capable 
of complete obliteration by the temporary approximation of its walls ; it is sometimes, 
however, dilated into a globular space, and sometimes spread out by a kind of semi- 
eversion into a nearly flat disc, which I have seen employed by the animal as an 
organ of adhesion. . ^ c \ 
(b.) Stomach.~~The stomach (fig. 3g) occupies the whole of the interior ot the 
clavate body of the polype ; as the endoderm passes backward from the post-buccal 
cavity to constitute the walls of the stomach, we find it forming a peculiar tissue 
composed of elongated cells, whose rounded ends project m prominent masses into 
the cavity of the stomach, where they constitute large irregular rugae. The tissue of 
these rugae is very remarkable; the cells composing it (fig. 4 i) are mother-cells, 
giving origin in their interior to several free secondary cells with distinct nuclei. In 
some of these secondary cells the contents are colourless and transparent, and the 
nuclei are then very evident (fig. 4 c) ; others contain a brown granular matter, 
by which the nucleus is more or less obscured (fig. 4 d) ; while others may occasion- 
ally be found filled with a brood of young cells (fig. 8). Lying apparently free in the 
mother-cells may also be generally seen, besides the free secondary cells, some rather 
irregular masses of a deep brown granular substance (fig. 4 e). I have found in Hydra 
a structure in all its essential characters entirely similar to this, and we cannot refuse 
to recognise in it an example of true glandular structure. The secondary cells are 
