HYDROIDA II 



chitinous sheath, which is extended to form the hydrotheca. Incipient hydrotheca formation is found 

 once or twice among the athecates, as for instance in Perigonimus, where a folding pseudo-hydrotheca 

 surrounds the polyp below the tentacle whorl. Less attention has perhaps been paid to similar for- 

 mations in a species such as Eudendrium vaginatum Allman, though even here we seem to find 

 trace of an incipient hydrotheca. In the thecaphores, however, the hydrotheca has developed into a 

 permanent component of the colony, and up to now, the systematic arrangement of the thecaphores 

 as a whole has been based mainly upon the hydrotheca and its features. In some few families such 

 as Haleciida, Plumulariida and Siliculariida, the hydrotheca is so small that the polyp cannot as a 

 rule be withdrawn entirely into it; often, indeed, the hydrotheca seems merely to furnish a support 

 for the basal part of the polyp. In view of the other structural conditions of the polyp, however, 

 this must not be regarded as a purely primary feature, but should apparently in most cases be con- 

 sidered secondary, as is shown more particularly by the two first-named families. Despite the fact 

 that the hydrotheca in Lafoeidcz, for instance, is far more highly developed, we must nevertheless, from 

 the structure of the polyp, consider the last-mentioned family as representing a more primitive stage 

 in the process of development 



A point of more importance is whether the ground plan of the hydrotheca is radial or bilateral. 

 This again is generally connected with the presence or absence of stalk. The stalked, and thus 

 free hydrotheca, will as a rule be constructed on a radial base type, as seen in Lafocidoe, Campanu- 

 linidcB, and Campanulariidce. The more or less marked curvings of the hydrotheca, not infrequently 

 found particularly in Lafoeida, do not suffice to efface the radial base type, and even in Lafoeida 

 and Campanulinidce, where the hydrotheca is sessile or even partly fused with the stolons, the radial 

 type is distinctly perceptible. In Siliculariida, on the other hand, the hydrotheca is assymmetrical or 

 bilateral, the cause of which peculiarity it is by no means easy to comprehend, since the hydrotheca 

 is quite freely set on its stalk, which is as a rule of considerable length. This family is, however, 

 altogether but imperfectly known up to now. The sessile hydrotheca has a greater tendency to devel- 

 op bilaterally. In the Plumulariida, this tendency is not yet quite pronounced; we find, however, 

 that the diaphragm in most of the species takes up an oblique position relatively to the longitudinal 

 axis of the hydrotheca. In Aglaopheniida, the development has proceeded considerably farther, the 

 diaphragm here being either markedly oblique or else generally falling into two obliquely set portions, 

 whereby the symmetry is arranged about a sagittal plane. This sagittal plane is often also found to 

 be the plane of symmetry for the hydrotheca as a whole, the arrangement of the teeth at the opening 

 being symmetrical according to the mentioned plane, while at the same time, a more or less marked 

 lateral compression is observed in the hydrotheca. A similar bilateral development of the hydrotheca 

 is encountered in Synthectid& and Sertulartida, where the diaphragm is as a rule also oblique, although 

 rarely, if ever, falling so distinctly into two halves as in several of the Aglaopheniidce. In Sertula- 

 rndce also, the distal parts of the hydrotheca show a distinct bilateral construction according to the same 

 sagittal plane as in the diaphragm. 



A secondary formation in the hydrotheca, the closing apparatus, is likewise of considerable 

 importance in systematic respects. In one genus of the Lafoeida, Toichopoma, we find a very primitive 

 type of closing apparatus; the lid here is simply formed by the one side of the distal end of the 



