HYDROIDA. 



The "Michael Sars" brought home some hydroids, 

 taken partly in hauls with a young-fish trawl, partly on 

 drifting sea-weeds. Though the collection is rather small 

 it is of great interest, contributing as it does to our 

 knowledge of the fauna of the subtropical eastern parts 

 of the Atlantic, and to our understanding of the origin 

 of the Sargasso fauna. 



Regarding the system employed in the present paper, 

 I must refer to the outlines given in my previous papers 

 on Arctic 1 ) and Adriatic 2 ) hydroids. The "Michael Sars" 

 material confirms the view defended in those papers: that 

 a system based on the development of the gonophores is 

 neither natural nor convenient, but only serves to disguise 

 the true relationship of the hydroids. 



A. ATHECATA. 



Perigonimus M. Sars. 

 Perigonimus Jonesi Osborn and Hargitt. 



Near Cape Bojador (Stat. 37) in 39 metres, many 

 fertile colonies growing on Nemertesla Hartlaubi. 



At first sight one might be inclined to refer colonies 

 of this species to BougainvilUa ramosa van Beneden, to 

 which they bear a close resemblance. A closer examina-, 

 tion, however, reveals the folded perisarc surrounding the 

 base of the contracted hydranths. The perisarc is very 

 delicate, and is almost invisible in preserved specimens 

 if the polyps are not contracted. 



B. THECAPHORA. 



THECAPHORA CONICA. 



Plumularia Lamarck. 



Plumularia setacea (Lin.) Lamarck. 



West of Gibraltar (Stat. 20) in 141 metres, many 

 fertile colonies growing on Nemertesla ramosa; Sargasso 

 sea (Stats. 64 and 67), some sterile colonies on floating 

 sea-weed. 



The fertile colonies from Stat. 20 were bisexual like 

 those from the Adriatic, 3 ) and the arrangement of the 

 male and female gonangia is the same, the former being 

 found on the basal parts of the stem, the latter on the 

 distal ones. 



Plumularia catharina Johnston. 



Sargasso sea (Stat. 67), some sterile colonies on 

 floating sea-weed. 



The ramification of the colonies is very interesting 

 (fig. 1). The two basal hydrocladia are opposite, whereas 

 the other branchlets are alter- 

 nating. The stem is divided 

 into irregular internodes. Two 

 or three basal internodes are 

 sterile, or only provided with 

 nematophores.. The next in- 

 ternodes are separated by 

 oblique nodes, and each of 

 them carries one hydrotheca 

 with its surrounding nemato- 

 phores; the lower of these 

 internodes is provided with 

 the basal parts of two oppo- 

 site branchlets, the others 

 having only the base of one 

 hydrocladium at the side of 

 the hydrotheca. The distal 

 parts of the stem are again 

 divided into heterogenous in- 



^ 



Fig. 1. Plumularia catharina 



from floating sea-weed at Stat. 67. 



(X 20). 



ternodes, every alternate one 

 being provided with a hydro- 

 theca and a hydrocladium, 

 the intermediate ones only 

 being furnished with nematothecae. 



At first sight we might believe that this mode of 

 branching would suffice to separate these colonies from 

 Plumularia catharina, which is generally said to have 

 opposite branchlets, and we find that the species of the 



') Die Hydroiden der arktischen Meere. (Fauna arctica, Bd. V). Jena 1909. 



■) Hydroiduntersucliungen III. Vergleichende Studien an adriatischen Hydroiden. (Det kgl. norske videnskabers selskabs skrifter 

 1911). Trondhjem 1912. 



3 ) Broch: Hydroiduntersucliungen III, p. 20. 



