HYDROIDA 



organised forms suggests, indeed, that we have before tis phenomena of convergency, the cause of 

 which is at the moment quite inexplicable. 



However, not only the ectoderm itself and its elements are of interest to comparative anatom>-. 

 The derivates of the ectoderm are of great importance. In the same way as the ectoderm of 

 the stem secretes a periderm, we find that with all thecaphore hydroids a chitinous hydrotheca is 

 secerned by the ectoderm of the polyp. Remarkably enough, a parallel is found also in a single genus 

 of the athecate hydroids, the Perigonhnits. In this genus the ectoderm of the polyp secretes a 

 "pseud oh yd ro theca", a hydrotheca-like, folding periderm case of a jellied .substance surrounding the 

 basal portion of the polyp up to the tentacle whorl. The first inquiries as to the pseudohydrotlieca 

 have been made by Hadzi (1913 and 1914). The pseudohydrotheca is di.stinguished from the real 

 hydrotheca in having no free margin, but being distally firmly connected with the ectoderm of the 

 polyp so as to be indistinguishable, on a superficial view, when the polyp is wholly distended. On the 

 contrary, when the polyp is contracted, the pseudohydrotheca is, in general, easily discerned, forming 

 a richly folded cover round the basal portion. The pseudohydrotheca bears some resemblance to the 

 genuine hydrotheca by the way in which the polyp is attached to it, the supporting lamniella of the 

 polyp being basally connected with the pseudohydrotheca by a whorl of small chitinous pro- 

 minences. Similar chitinous prominences are also seen, for instance in EHdendrhim, connecting the 

 soft parts of the stem with the periderm cover; systematically, however, no particular interest can be 

 attached to them. 



Also in the endoderm diversities of great interest are found. The simplest, most homogeneous 

 shape is represented by the gastric endoderm of Clava (Broch 1911), forming a homogeneous epithel- 

 ium for absorbing the nourishment, from the orifice of the mouth to the passage of the polyp into 

 the stem; almost all of the cells of the gastric endoderm are filled with larger or .smaller grains 

 showing a strong affinity to Dela field's hsematoxyline ("nutritive cells" and "albumen cells", comp. 

 Schneider 1902). As to Corync, the state of things is quite different; here the endoderm in the 

 portion nearest to the mouth is extremely rich in mucous gland cells, while the digestive cells are 

 comparatively few in number. In Corync we must consequently distinguish between the oral 

 portion secerning mucus and the part of the endoderm of the polyp which is the proper gastric or 

 digestive portion. The difference between these two endodermal zones appears still more distincth- m 

 Myriothcla; the glandular cells are here denseh- concentrated on a small portion near to the moutli, 

 strongly conspicuous by its clear blueing after being treated with Delafield's hsematoxyline; the 

 other endodermal cellular forms have almost wholly disappeared in the glandular zone with 

 Myriothela. In Tubularia, on the contrary, the glandular zone has disappeared, so that the endoderm 

 here by its homogeneous appearance all over the pohp strongly recalls the case of Clava. 



A rather different state of things is found in the Botigainvilliida'. Here, indeed, mucous cells, 



occur in the oral endoderm of the polyp. But the bulk of the cellular elements in the oral portion as 



far as the whorl of the tentacles, is constituted by cells which appear indifferent to the nutritive elements. 



All the cells here have small nuclei strongly concentrated, while in the gastric endoderm taking the 



nourishment, from the whorl of tentacles and downwards, the nuclei are large, with open chromatme 



net-work. This condition of things is still more pronounced in Etidnidrium , the mucous cells of 



2 



The Ingolf-Expedilion. V, 6. 



