34 AMERICAN HYDROIDS. 



In several species for example, Aglnopheniti iubulifem liincks aud A. insii/nix Fewkes (Plate 

 XIX, tig. 7) there is an accessory leaf hanging downward aud outward from near the base of the 

 proximal leaf. Its significance is not known, but it is interesting in that it shows the two rows of 

 neniatophores normally present, the proximal one of which is hidden, in the other leaves, by the 

 imbrication of adjacent leaves. 



In all gonosomes with true corbuhc the gouangia are borne at the bases of the leaves, there 

 being normally as many gouangia as there are individual leaves. There is usually in each leaf 

 a longitudinal transparent strip, sometimes widening into a broad band or oval space. It is so 

 perfectly transparent in some cases as to lead one to believe that it is an open space cut out of the 

 leaf. By a proper management of the light, however, or by staining, it can alw ays be demonstrated 

 to be a thin plate of chitine. It is of interest as suggesting a means by which light may be 

 admitted to the growing planulie within the corbula. It is hard to conceive any other use for these 

 delicate windows in the corbula- leaf. 



In the genus Tliccocarpiis a true corbula, although a distinctly different one from that just 

 described, is found, in which the corbula-leaves are very narrow, saber-shaped, aud widely 

 separated, each bearing a row of neniatophores along one edge and a hydrotheca on its basal 

 portion (Plate XXIV, figs. 15 and 10'). The hydrotheca has its anterior side applied to the modified 

 corbula-leaf, and behind it is a small tubular structure, representing the cauline iuteruode, aud two 

 supracalycine neniatophores in their normal position. Professor Allman considers this form of 

 corbula a beautiful demonstration of his theory that the corbula-leaves are merely immensely 

 developed mesial nematophores, and indeed no one could study these structures without being 

 strongly impressed with this view. As already indicated, however, my own impression is that 

 hydranths and sareostyles are homologous, or, as it were, interchangeable structures. Indeed, we 

 might go still farther aud suggest that all ''persons' 1 of the hydroid colony are fundamentally 

 homologous and often interchangeable; that the hydrauth, sarcostyle, and blastostyle are each to be 

 considered as a special modification of the primitive hydroid polyp. There are many facts which 

 indicate that each of these maybe converted into either of the others. For example, the hydrauth 

 may become a nematophore. as observed by Mere jkow sky in Flumularia luilecioidex; ' the hydrauth 

 maybe converted into a blastostyle, as in .Eudendriinn aud other gymnoblastic forms; a given 

 structure may serve as both blastostyle and hydranth body at the same time, as in the remarkable 

 hydrauth bearing gouophore of Iltilccium lialecinum; and hydranths may apparently be, replaced 

 by blastostyles, as in the genus Syntlieciinn of Allman. Nor does this remarkable power of inter- 

 changing of persons in the different parts of the hydroid organism end here, for 1 have seen the 

 terminations of hydrocladia in Plumularia pinnata, which, as in all plumularians, bear developing 

 hydranths, change into rapidly growing stolons, which ultimately develop new colonies.- In view 

 of such facts it would seeni, as I have already suggested, unnecessary to devote much time to 

 attempt to homologize these various parts with the corbula leaves of TJiccocarpnis. 



Phylactocarps which appear to be modified branches are met with among the Statoplea in the 

 genus Lytocarpus. Dr. J. Walter Fewkes, in working over the Blake material, found specimens 

 which were characterized by the possession of a sort of pseudo-corbula which differed from the true 

 structure in the fact that it was a modified branch instead of a modified hydrocladium. For this 

 form he instituted the genus 1'leurocarpn. In the present work I have deemed it necessary to 

 include this form, with several allied ones, in the genus Lytwarpux. The pseudo-corbula is formed 

 by the hydrocladia on a certain portion of a branch being replaced by neniatophorous appendages 

 which are borne alternately on the rachis and curve outward, upward, and inward, so as conjointly 

 to form a structure looking much like an open corbula (Plate XX XII, fig. 2). The, long tabular 

 neniatophores are arranged regularly in sets of three and are supposed to represent the supra- 

 calycine and mesial nematophores of suppressed hydranths. There is a hydrotheca at the base of 

 each of these appendages which has all three nematophores, and above this one or more gonangia 

 are found, each with three nematophores around its point of origin on the protective branchlet, 

 thus giving evidence that the gonophores hero replace hydranths (Plate XXXII, figs. 3, 4). The 

 distal portion of the branch is unmodified, bearing regular hydrocladia. In some specimens which 

 I have examined there is evidence that the protective appendages are actually metamorphosed 



1 Archives <le Xoologie Experimental <-t (ionoraie, X, 1882, p. 607. - American Naturalist. November, 1895, p. 966. 



