8 ARTHUR DENDY. 



internal tissues is not all that could be desired, and it is 

 greatly to be hoped that an opportunity may arise of working- 

 out this subject more in detail with the aid of material 

 specially treated for the purpose. It is also highly desirable 

 that a detailed comparison should be made of the histological 

 structure of Corymorpha, Monocaulus, and Branchio- 

 cerianthus, which are evidently related to Pelagohydra, 

 and, like it, of exceptional size. 



Wall of the Proboscis. — The ectoderm is a thick layer 

 densely charged with small, darkly staining nuclei and 

 thread-cells irregularly scattered throughout its substance 

 (figs. 9, 10, EcL). 



In section it exhibits numerous fine radial lines running 

 in at right angles from its outer surface, and perhaps 

 indicating the boundaries of a, single layer of large prismatic 

 cells. On its inner aspect, immediately contiguous to the 

 mesoglcea, is a well-developed layer of longitudinal muscle- 

 fibres. In transverse sections (ng. 10) we see that this layer, 

 consisting of an approximately single row of fibres, is thrown 

 into longitudinal folds, the mesogloea being produced out- 

 wards in plate-like ridges between the folds. This arrange- 

 ment, so well known in the mesenteries of the Actinians, no 

 doubt serves to inci-ease the extent of the muscular tissue. 



The ectoderm decreases in thickness from below up- 

 wards, and the folding of the muscular layer is especially 

 conspicuous just above the region of the tentacles, and dies 

 away as it approaches the upper limit of the proboscis. 

 Between the bases of the proboscis tentacles the ectoderm is 

 extremely thick, but thins out greatly over the tentacles 

 themselves. 



The endodermal lining of the proboscis wall is enormously 

 thick, and throughout the greater part of its extent is thrown 

 into prominent longitudinal folds or ridges in the manner 

 already described (figs 5 — 9). The structure of these ridges 

 (figs. 9 — 11) is very peculiar. The mesoglocal supporting 

 lamella which divides the endodcrm from the ectoderm is 

 not continued into them, and is indeed sharply marked off by 



