REPORT ON THE HYDROIDA. 
IX 
greater part of the thickness of this endodermal layer, consist of round nucleated masses 
of clear protoplasm, while those which lie directly on the walls of the cavity are ciliated, 
and overlaid by a thin layer of free protoplasm, through which the cilia pass, and 
which has the faculty of emitting pseudopodia. 
In Tubularia indivisa also, and in Corymorpha, the endoderm of the stem is com- 
posed of many layers of round cells. These would form a solid mass, filling the cavity of 
the stem in these Hydroids, were it not that the endoderm is here traversed by numerous 
longitudinal canals, which run parallel with the axis, anastomosing with one another here 
and there, and opening above into the gastral cavity of the hydranth. These canals 
represent the simple cavity which extends through the axis of the stem in other Hydroids, 
for there is here no axial cavity in the stem, its place being taken by a large and 
clear-celled modification of the more peripheral portion of the endoderm. 
In Monocaulus imperator the endoderm of the stem is traversed as in Tubularia 
indivisa and in Corymorpha by longitudinal anastomosing canals (PI. III. figs. 1, 4); 
but here a wide, continuous cavity runs through the whole length of the axis. 
The nutritive cavity of the Hydroida is generally dilated in the body of the hydranth, 
so as to form the proper stomach, and then the body of the hydranth with its included 
cavity contracts towards the mouth into a conical, or in some cases, a trumpet-shaped, 
proboscis or hypostome. 
In many species of Tubularia and other Hydroids which form the section 
Gymnoblastea, the endoderm of the hypostome is thrown into longitudinal ridges, mostly 
four or five, which project into its cavity, and then passing downwards become in the 
stomach broken up into several branches, which soon lose themselves on the general 
endodermal lining. 
These endodermal ridges, which are also well developed in the nearly allied order of 
the Siphonophora, were first pointed out by von Koch, 1 and have been since noticed by 
many observers, but more especially by Hamann, 2 who adopted for them Haeckel’s term 
“ tseniola,” a term, however, wdiich Haeckel uses in a different sense, applying it to the 
prominent gastral ridges which are characteristic of the Scyphistoma or polyp form of 
the Acraspedal Medusae. 3 Hamann attributes to the endodermal ridges of the Hydroida 
a high systematic value, and regards their presence as the true grounds on which the 
Gymnoblastic Hydroids admit of being separated as a natural group from the Calypto- 
blastic forms. 
This, however, is assigning to them an importance greater than they can fairly claim. 
They are by no means absolutely constant either in their form or their presence in the 
Tubularians. In Tubularia indivisa they are represented by pendulous pyriform lobes 
1 G. v. Koch, Vorlaufige Mittheilungen iiber Colenteraten, Jenaische Zeitschr., Bd. vii. p. 512, 1873. 
2 0. Hamann, Der Organismus der Hydroidenpolypeii, Jenaische Zeitschr., Bd. xv. 
3 Ernst Haeckel, Das System der Medusen, Jena, 1879. 
(ZOOL. CHALL. EXP. — PART LXX. — 1888.) 
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