I. Comparative anatomy of the nourishing individuals, and 

 systematic division of the thecaphore hydroids. 



As with the athecate h\-clroids, so also in the case of the thecaphores, the comparative anatomy 

 of the nourishing individuals affords us a certain working basis in systematical respects. With regard 

 to this group, however, the investigations are still somewhat incomplete, as will be seen from the 

 following. 



The ectoderm of thecaphore hydroids is apparently very uniform in point of development, 

 and the nematocysts, which in the athecates furnished good material for study, seem in the theca- 

 phores to vary but slightly, and should on the whole be referred to the same type as in Athecata 

 ftlifera. This point is of some considerable interest as further confirmation of the old theory as to a 

 closer relationship between the thecaphores and the mentioned group than between the thecaphores 

 and Athecata capitata. Even among the Gramiiiaria^ where the nematocysts, in one species at any 

 rate, are dimorphously developed, we can find no resemblance to the capsule form in Athecata capitata. 

 The arrangement of the stinging cells also is very uniform in the thecaphores. As a general rule, 

 we may say that the nematocysts in the thecaphores appear in marked transverse zones about the 

 tentacles; the only exception I have found here is that of the gigantic Bonneviella polyps, where the 

 zonate arrangement has become effaced. 



A somewhat different organisation of the ectoderm is encountered in one or two families. The 

 grown polyps of Syntheciidw and Sertidariidcr have developed adhesive lamella, an exterior ectodermal 

 lamella covering the inner side of the hydrotheca for a greater or lesser extent when the polyp has 

 withdrawn into the same. It is this ectodermal lamella which Nutting (1904 p. 10) considers as one 

 or more "protractors", evidently from more or less accidental breaks in the lamella, this being, in the 

 living polyps, continuous. Nutting even goes so far as to base part of his system upon the number 

 of "protractors", and is here followed by Broch (1905, 1909). These protractors, however, form, as 

 pointed out by Kiihn (1913 p. 66) an unbroken sheath or covering on the inner side of the hydro- 

 theca, attached at its basal margin to the body of the polyp, and at the distal to the opercular appa- 

 ratus. A similar arrangement is also found in Aglaopheinidcr; here likewise we encounter lamellous 

 extensions of the ectoderm, attaching the polyp to the inner ribs of the hydrotheca. 



We see then, that the ectoderm itself gives us very little to go upon in a systematic classifi- 

 cation. On the other hand, its derivates, i. e. the periderm, will in the thecaphores be found of great 

 importance in this respect. The thecaphores are, as we know, characterised by their stiff, almost 



The Ingolf-ExpcJition. V. 7. I 



