248 Prof. E. Hackel on the Position of the 



If we compare the coarser and finer structural characters of 

 Hydra and Gordylophoray as these appear to be established by 

 the extremely careful histological investigations of Kleinen- 

 berg and F. E. Schulze, with the corresponding structural 

 characters of Olynthus, we cannot but be astonished at the 

 remarkable agreement which is manifested even in the finer 

 details. This agreement appears most striking when we 

 consider the Olynthus with closed pores ^ or Prosycum, or if we 

 leave out of consideration the calcareous spicules, the group- 

 peculiarity of the Calcispongise, and take, instead of Olynthus^ 

 the Archispongia (which differs only by the absence of spi- 

 cules). As essential agreements of structure between Hydra 

 and Cordylophora on the one hand, and Prosycum and the 

 Archispongia on the other, we have : — 1 , the simple sto- 

 machal cavity with a buccal orifice ; 2, the composition of 

 the thin stomachal wall of two laminae, the vibratile entoderm 

 and the non-ciliate exoderm ; 3, the composition of the ento- 

 derm of flagellate cells. 



On the other hand, we have as essential differences : — 1, 

 the constitution of the exoderm, the cells of which in Hydra 

 and Cordylophora develop urticating capsules and neuro- 



because, of all the accurately known forms of this gi'oup, I regard them as 

 the simplest and most primitive, and as most nearly approaching the 

 imknown common stock form of the whole group, the hj^othetical 

 Archydra. It is true that in 1870 Richard Greeif described, under the 

 name of Protohydra Leucharti, a form apparently still simpler — namely a 

 hydroid polype xvithout tentacles, and which is said to propagate by mere 

 transverse division (Zeitschr. fiir wiss. Zool. 1870, Bd. xx. p. 37, pis. 4, 5). 

 GreefF represents it as " a marine stock form of the Crelenterata," as an 

 " undoubtedly completely developed and mature, but asexual animal form, 

 propagating by transverse division." But from his whole repi-esentation 

 it seems to me, on the contrary, to follow indubitably that here we have 

 to do with an imperfectly developed hydroid form, which will subse- 

 quently become sexually differentiated. It would be contrary to all 

 analogy that an animal form so highly differentiated, which in its 

 essential anatomical structure seems to agree exactly with Hydra, and 

 differs therefrom only by wanting tentacles, should propagate merely 

 asexually by transverse divmon. The question would be very different if 

 Protohydra propagated asexually only hy spores (or single separated cells). 

 At any rate Greefi's assumption that Protohydra, which was observed 

 " for a couple of months" in an oyster-park at Ostend, is undoubtedly an 

 independent hj^droid form is quite unjustilied. Greeff says, "On a 

 careful examination of its whole habit, its structure, and movements, and 

 taking into consideration its transverse division, and above all the long 

 period of observation, all notions that it is a developmental form of an 

 Anthozoon or any other form of animal, or of a hydroid polype developed 

 and mature in its asexual stage, must disappear." These arguments, 

 however, prove nothing at all ; and these rejected notions will only be 

 clearly established in the mind of an unprejudiced reader by Greeff 's OAvn 

 representation. So long as the developmental history of Protohydra is 

 completely unknown, we need take no notice of this hydroid form. 



