Systematic Studies on the Sertulariidae. 
By 
G. M. R. Levinsen, 
(With Plates IV—V). 
In 1893 I published aåa paper on the Medusae, Ctenophores 
and Hydroids from the west-coast of Greenland, in which I made 
an attempt to introduce a more natural arrangement of certain 
families of Hydroids, advocating the view, that in the limitation of 
the genera the characters found in the single individuals of the 
colony, and especially in the trophosomes, ought to be preferred 
to such characters, which might be derived from their different 
arrangement on the stems and branches of the colony.: As to the 
Sertulariidae I proposed a limitation of the genera on the basis 
of the diversities presented by the opercular apparatus, and by the 
margin of the hydrotheca, the form of which is always contingent 
upon the structure of the operculum. While a number of authors 
(Marktanner-Turneretscher, Schydlowsky, Broch, Sæ- 
mundsson and Kramp) have followed my view, it has been 
attacked by others, but before I undertake to answer the objections 
raised against it, I shall set forth some general remarks on the 
systematic value of the colonial form or the form of growth. 
In all aggregate animals we have to discern between two cate- 
gories of characters of very different systematic value, namely those 
presented by the single individuals (the zooidal ch.) and those 
derived from the different ways in which they may be arranged in 
the colony (the zoarial or colonial ch.). In the first attempts at 
a systematic arrangement of aggregate animals the latter characters 
