I. Introductory Notes. 



Scarcely in any other part of the animal world greater difficulties are thrown in the way of 

 the systematist than those occasioned by the lowest Coelenterata, impeding the attempts to establish 

 a natural grouping of the hydroids and their attendants, the hydromednsae. This is due not only to 

 the actual deficiencies of our knowledge of these low organisms, but also to the fact — as pointed 

 out by Kiihn in his excellent summary (1913) — that the groups present partly a confused series of 

 adaptations and phenomena of convergency, partly the occurrence of medusae of widely divergent 

 development as companions of closely allied hydroids or, vice versa, closely allied medusae accompanying 

 hydroid polyps divergently developed. It is a striking fact that in certain species, for instance the 

 Codomdcr, the phylogenetic development and differentiation of form of the medusae have been com- 

 paratively stunted, whereas the polyps {Corynidce, Pcnnariidcr, Tubulariidir) have developed hetero- 



geneously so as to present a large series of various forms. The exact reverse is represented 

 by the homogeneous polyps of the family Bougainvilludcr. forming the nurse generation of the hetero- 



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geneously developed hydromedusa: of the families Margelida and Tiaridce. The possibility of construct- 

 ing a safe system common to medusas and hydroid polyps, indeed, appears remote. For in the first 

 place the two "generations", on account of their excessive abundance of species, have had to be treated 

 separately, each generation by specialists of its own, and moreover, we are in fact still in the dark as 

 to which of the two generations is to be regarded as the primitive or the phylogenetically older. 



When looked upon as a whole the group of Zoophytes must be characterized as a very low 

 group of animals. In the nurse generation as well as in the free-swimming medusae the structure 

 of the individual is very simple, though the medusae after all must be said to be a little higher 

 organised than the polyps. In addition to the two primary layers of cells, the endoderm and the ecto- 

 derm, occurs in the medusae a "mesoglcea", whose descent from one or the other of the primary layers 

 is not yet definitely ascertained as far as all species are concerned. However, a mesoglcea also occurs 

 with some hydroids, for instance the "parenchyma" of the Tubitlariidcc, which is a typical endodermal 

 formation. 



To students of the hydroids it very soon becomes obvious that the leading systematic characters 



have been derived from such criteria as urge themselves on a superficial view of the animals, while, 



contrarily to the method practised in investigating most other groups of animals, the inner 



anatomy is all but entirely left out of account. This is due to the generally accepted view that the 



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