HYDROIDA 



organised forms suggests, indeed, that we have before lis plienoniena of convergency, the cause of 

 which is at the moment quite inexpHcable. 



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

 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 b}- the ectoderm of the i^olyp. Remarkably enough, a parallel is found also in a single genus 

 of the athecate hydroids, the Pt'rif;oiiimus. In tliis genus the ectoderm of the polyp secretes a 

 "pseudohydrotheca", a hydrotheca-like, folding periderm case of a jellied substance surromiding the 

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

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

 hydrotheca in having no free margin, but being distalh' 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 lammella of the 

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

 minences. Similar chitinous ]3rominences are also seen, for instance in Eudcndriiiiii, connectmg 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 Delafield's hsematoxyline ("nutritive cells" and "albumen celLs", comp. 

 Schneider 1902). As to Coryiu\ 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 nunil)er. In Coryuc we must consequently distinguish between the oral 

 portion secerning mucus and the part of the endoderm of the poh'p which is the proper gastric or 

 digestive portion. The difference between these two endodermal zones appears still more distinctly in 

 Myriotlicla\ the glandular cells are here densely concentrated on a small portion near to the mouth, 

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

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

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

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



A rather different state of things is found in the Bongaimnlliidg: Here, indeed, mucous cells, 

 occur in the oral endoderm of the polyj). lint 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 element.s. 

 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 chromatinc 

 nrt-work. This condition of things is still more ])ronounccd in Hitdiiidriidii , the mucous cells of 



riic Inyolf-lixncilition. V. 6. 



