AGASSIZ. NATUEAL HISTOEY OF T1IE UNITED STATES. 347 
he directed attention, in a marked manner, to the striking peculi¬ 
arities of the PEginidce, a group of naked-eyed Medusae at that 
time imperfectly appreciated. 
Meanwhile the development of the DiscopJiorce was being success¬ 
fully investigated by a number of able observers, and, among other 
singular phenomena thus brought to light, it appeared that many of 
the Cryptocarpae were but the free-swimming reproductive buds of 
various fixed Hydroids and Siphonophora. It was shown also, alike 
by the method of gradations and the test of development, that the 
structure of these highly specialized zooids passed by imperceptible 
degrees into that of other fixed and lower organized generative 
bodies belonging to different members of the same groups. 
With regard to the Phanerocarpce , it was proved that some, at 
least, of these creatures resulted from the transverse fission of asexual, 
hydraform larvae, or rather protozooids, very minute in size com¬ 
pared with their large free-swimming reproductive bodies, but, 
though less complex, decidedly resembling them in structure. The 
ova of these zooids, when fertilised, become developed into ‘ Hydra- 
tubse,’ which again, by fission, produce “jelly-fishes.” 
The Hydra-tub se, both in form and organisation, also closely 
resemble the marine animals referred by O. F. Muller to the genus 
j Lucernaria (family Lucernariadce of Johnston). This view of the near 
affinities of the Lucernarice was, in 1856,* distinctly enunciated by 
Huxley. It is true that, at an earlier date, they had been placed, 
conjecturally, in the neighbourhood of the Medusae , but most 
naturalists still continued to associate them with the Sea-Anemones. 
Huxley, however, went so far as to unite the Lucernarice and Phanero- 
carpae into one order, which he at once termed Lucernaridae. The 
Craspedota he divided, potentially, into two groups : viz. (1), free 
reproductive zooids of other orders of Hydrozoa , and, therefore, 
rightly referrible to such orders ; and (2), those “ developed directly 
from the eggs of similar organisms.” The existence of these last he 
regarded as probable, not proven. For their reception, as also for 
that of those Craspedota “with whose origin we are unacquainted,” 
he established a provisional order, Medusidae. f 
But Kolliker, in 1853, J relying chiefly on the observed differences 
between their modes of development, had separated the Cryptocarpce 
from the higher Discophorae , a term which he restricted to the 
Phanerocarpae or Steganophthalmata .|| 
We come at length to the views of Professor Agassiz, as expressed 
in the volume before us. The Discophorae , in his opinion, constitute 
one of the three orders of the class Acalephce in its widest sense (the 
* Lectures on General Natural History, (Lecture IY.) Medical Times and Gazette, 
June 7. 
f The Oceanic Hydrozoa, 1859 ; p. 21. 
J Die Schwimmpolypen oder Siphonophoren von Messina, p. 77. 
|| Leuckart goes further than this, and even uses the word Acalcphce as a syno¬ 
nym of Phanerocarpce (Sec his Bericht in Wiegm. Arch.). 
