3 AMERICAN HYDKOIDS. 



The hydrothecal walls are usually quite thin and hyaline and often easily collapsible; but 

 in Ortlwpyxis (lig. 12) and Silicularia (fig. 8) they are often enormously thickened, so much so 

 that in the latter genus the hydranth is incapable of retracting within the lumen of the hydro- 

 theca. In tliis latter genus also we meet with the pecuUar bilaterally symmetrical hydrotheese 

 referred to above, in which the upper part is cut away in such a manner that the long axis of 

 the margin is inclined at an oblique angle with the axis of the hydrotheca, the wall being much 

 lower on one side than on the other. The excessive thickening of the hydrotheca in Orthopyxis 

 caused a cxirious but natuial misconception on the part of some of the earlier wiiters, partic- 

 idarly Hincks,^ who believed that these hydrotheese were double, one cup being set within 

 the other, an optical effect due to the almost complete transparency of these structures. 



There is often a great degree of variation in the amount of thickening of the hydrothecal 

 walls in the same species and even in the same colony, as in Orthopyxis compressa (figs. 12, 13), 

 in which some of the hydrotheese are Uttle thicker than in ordinary campanularians, while others 

 are so thick as to be almost orbicular in outline. In Silicularia hemispherica (AUman) (fig. 

 17), the hydrotheca is shortened so that the lumen is hemispherical or bowl-shaped. 



In a number of species the distal portion of the hydrothecal wall is longitudinally pleated, 

 each pleat corresponding to one of the hydrothecal teeth. This is well shown in Campanularia 

 magniiica (fig. 21), where the longitudinal folds may reach nearly to the hydrothecal base. 



The margin of the hydrotheca is perfectly even in many species of Obelia and less com- 

 monly so in Campanularia, this condition being found occasionally in practically all genera. 

 The toothed margin is also common throughout the family, the teeth varying from hardly 

 evident marginal undiilations {Ohelia duhia) (fig. 19) to slender thornhke ]ioints, as in GaTrir- 

 panularia ptychocyaihus (fig. 22) or Gonothyrsea gracilis. In some cases the teeth are bimucro- 

 nate, as in Obelia austrogorgia (fig. 14) or 0. hicuspidata, while in others they have squared-off 

 or truncated ends, as in Campanularia hinchsii (fig. 11). 



One of the most remarkable modifications of the margin is found in Can^panularia retro- 

 flexa AUman (fig. 16), in which "the margin is everted in a plane at right angles to the axis of 

 the hydrotheca, and the teeth into which the rim is divided stand up from it parallel to the 

 axis, thus suggesting the form of the escapement-wheel of a watch." ^ 



In some of the more delicate species the upper part of the hydrotheca is so thin-walled as 

 to be collapsible, the teeth bending inward and forming a sort of pseudo-operculum, as in 

 Campanularia ptyclwcyaihus AUman (fig. 22), while in others the rim is reinforced by a cir- 

 cular band of chitin, as in Obelia marginata (fig. 23). In Obelia striata Clarke the distal ])art of 

 the hydrotheca is deeply fluted, the margin being broken up into a series of horizontal semi- 

 circular folds between wliich are inward-projecting teeth or vertical crests,'' and in Campanu- 

 laria (?) obliqua Clarke the marginal teeth are regularly pointed obliquely like the teeth of a saw.* 



Hie diaphragm. — This structure has given rise to much discussion of late, mainly because 

 of Levinsen's attempt to base a generic classification of the family Campanularidas upon the 

 characters of the diaphragm. He says: ^ 



In the species that have a creeping stolon, or whose stem is composed of a fascicle of parallel branching tubes, the 

 diaphragm is composed of two different parts, namely, partly of a strong and ring-like process from the hydrotheca 

 and partly of a thin chitinous membrane springing from the upper edge of this ring and which is secreted by the lower 

 surface of the basal part of the hydranth and is plainly visible in an entirely empty hydrotheca. In all such speci- 

 mens as are provided with a free branched stem the diaphragm never presents such a difference between an outer and 

 an inner portion. This consists of a fairly solid horizontal chitinous plate which, as a rule, has the same thickness 

 throughout. 



Tlie above translation is by Mr. J. H. Paarmann, to whom the writer is under obligation 

 for his painstaking sectioning of many species of Campanularidse for the purpose of studying 



1 British Hydroid Zoophytes, 1868, p. 165. 



= Challenger Eeports, Hydroida, part 2, 1888, p. 21, pi. 11, fig. la. 



= Memoirs Museum of Comparative Zoology, vol. 35, No. 1, 1907, p. 9, pi. 7, fig. 4. 



^ Idem, pi. 3, figs. 2, 3, and 4. 



^ Meduser, Ctenophorer og Hydroider fra Gronlands Vestkyst, 1893, p. 160. 



