PREVIOUS INTERPRETATIONS OF CNIDARIA 107 



Hydra was considered as a prototype of all Cnidaria; it usually 

 appears solitary, and it forms only small ephemeral cormi (co- 

 lonies), a phenomenon which is usually interpreted as a ten- 

 dency towards the evolution of colonies. In spite of the fact that 

 it can now be shown to be proved that the solitary character 

 of Hydra is a secondary phenomenon and that it is far from 

 being a primitive form, there are even now numerous zoolo- 

 gists who find it difficult to accept this fact. An interesting 

 parallel to Hjdra can be found in the species Monohryo^oon 

 amhtilans Remane (Remane, 1936), which until recently has been 

 thought to be the only solitary ectoproctan (recently, however, 

 another such species has been described); yet in this case the 

 only difference has been that the discoverer of Monobryozoans 

 had immediately been able to estabHsh that in this species the 

 solitary state was a secondary phenomenon. This was not diffi- 

 cult to estabUsh in view of the fact that in this form it shows 

 that it goes back to corm-forming ancestors. The number of 

 solitary species of Hydroidea is comparatively high. A closer 

 comparison with the related species or genera enables us to 

 discover again and again that this solitary life is a secondary 

 phenomenon. Not infrequently we can find all kind of transi- 

 tions, from species with typical cormi (colonies), and those 

 that, though solitary, still show, clear traces of a former for- 

 mation of cormi, up to the completely solitary species (e.g. 

 in the hydroid family Tubulariidae). The last remaining trace 

 of a former formation of cormi is the formation of podocysts 

 (Had^i, 1912), i.e. the parts of coenosarcs or of the cellular 

 mass w^hich are covered by the periderm (i.e. cuticule) and which 

 otherwise form the impersonal part of a cormus, above all 

 the hydrorhiza. In our case they take over the role of a resting 

 stage and serve the purposes of asexual reproduction (Fig. 19). 

 As early as in the Anthozoa, as well as in all the truly sessile 

 animal types up to Chordoma, we can observe an inclination 

 either to an imperfect transfission or to the formation of buds 

 with a simultaneous preservation of an organic contact be- 

 tween the secondary zooids and the maternal polyp. Thus we 



