36 



HYDROIDA II 



creeping stolons. This thus confirms, as Kranip points out, the doubts which have been expressed 

 by certain previous writers as to the fundamental importance of the structure of the colony for hydroid 

 systematics, and partly effaces the limit between creeping species and species with upright, composite 

 colonies. 



The genus Halecium is characterised by the lively renovation of its hydranths. This should, 

 it would seem, be explained as due partly to the lack of special defensive individuals, partly to the 

 minimal size of the hydrothecse, and finally also to the large dimensions of the polyps, which probably 

 render them particularly attractive to creatures preying upon hydroids generally. The marked reno- 

 vation activity often leads to the formation of whole piles of hydrothecse, or apparently small branches, 

 for which Schydlowski (1901) has introduced the term "ps,eudohydrocauli". 



Not infrequently also, heteromorphotic renovates may be observed; I have previously (1909 p. 

 151) noticed that the formation of gonothecse as heterorenovates is characteristic of the male in Hale- 

 cium ornatum Nutting. Heteromorphotic renovates of a more accidental nature will be noted later 

 on in several species; these are of great interest, since renovates of this kind have, according to 

 Had/.i (1915) only been met with in nature among HalfciiDii and Syiitkeciidir. The question has, 

 however, been too little studied as yet to permit of our drawing further conclusions from this, but 

 the point should be kept in mind, until it has been fully cleared up. 



Halecium halecinum (Liune) Oken. 

 1758 Sertularia halecina, Linne, Systema Naturae, Ed. 10, p. 809. 

 1815 Halecium halecii/Kiu, Oken, Lehrbuch der Naturgeschichte, vol. i, p. 91. 



Stiffly built, as a rule doubly pinnate colonies with polysiphonic main stem. The branches 

 are as a rule piunatel)- ratnified in the same plane as the main stem, or the colonies may, more rarely, 

 assume an irregular bushy shape. The minor branches are divided up into regular internodia, the 

 length of which is about twice the distal breadth. The primary hydrothecae are low, often almost 

 like a mere opening in the branch apophysis at the distal end of the internodium. The secondary 

 hydrothecse are small, with a large basal cavity, generally somewhat asymmetrically developed, and 

 having the basal part of its adcauline wall highly concave. The hydrothecse are somewhat broader 

 at the aperture than at the well-developed, fairly strong diaphragm; the opening margin is not 

 curved outward. 



The gonothecse are large, and proceed from the apophysis at the base of the primary hydro- 

 thecse. The males are narrowly c>lindrical to elongated oval, tapering below, distalh" cut off trans- 

 versely, or more often broadly rounded. The female gonothecse have a distal laterally placed aperture 

 with a pair of hydranths; the aperture is furnished with a short cylindrical neck, and situate normally 

 at the distal end of the hydrothecse; more rarely, the distal part of the gonotheca may be domed 

 somewhat forward, so that the opening is a little below the point. The gonotheca is slender, almost 

 entirely straight or slightly convex on the side opposite the pair of hydranths, and tapering evenly 

 downwards. 



