240 M.C. Mereschkowsky on the Hydrotda. 
and which may be generalized into a single idea, a. single 
law of metamerism (Metamerengesetz) or of articulation. This 
law may be formulated as follows:—The Hydroid may be 
composed of two or several metameres, simiiar or not ; each 
metamere in its turn is composed of several antimeres. 
Fig. 1 shows diagrammatically a Hydroid belonging to 
what I call the articulate type; it will be seen that it con- 
sists of three very distinct metameres, each of which is in 
its turn composed of four antimeres*. 
This law governs a considerable number of forms among 
the Hydromeduse ; we may recognize it in the species be- 
longing to various genera—for example, in Stawridium, Coryne, 
Fig. 2. 



Syncoryne, Millepora, Cladonema, Tubularia, Cordylophora, 
Gemmaria, &c. ; but all the cases in which we remark the arti- 
culate type among the Hydromeduse belong exclusively to 
the order of naked Hydroids (Athecata). There is not a 
single Hydroid belonging to the order Thecaphora which 
* According to M. E. Hackel we should have to call such a type 
T3 *, . . ” 7 
‘forma staurastoma diplopola articulata, 
