DISTRIBUTION OF HYDROZOA., vel 
found in Northern Europe, being circumpolar in their range. 
A distinct assemblage of Sertularians, characterized by the 
large number of species of Plumularia, inhabits the Florida 
seas down to a depth of five hundred fathoms. Among 
the Discophora the Lucernarie are arctic as well as temper- 
ate forms, while Cyanea is peculiar to the Northern Hemi- 
sphere. Aurelia and Pelagia are cosmopolites, while Rhaco- 
pilus, Placois, and Lobocrocis are peculiar to the Southern 
Hemisphere. The larger number of species are tropical and 
sub-tropical. As regards their bathymetrical distribution, 
while several species extend to the depth of five hundred 
fathoms, Monocaulus flourishes in gigantic proportions at 
the enormous depth of four miles. 
The range in geological time of the Discophora extends 
to the Jurassic period (middle Oolitic), large species of jelly- 
fishes occurring in the Solenhofen slates. The genus Hy- 
dractinoa first appeared in the Cretaceous period. Grapto- 
lites were common in the shales of the Potsdam period, so 
that if Graptolites are Acalephs, the latter are probably as 
old a type as any, being contemporaneous with trilobites, 
brachiopods, mollusks, worms and sponges. 
Crass IL—THE HYDROZOA. 
Body in its simplest form a sac attached by the aboral end, composed of 
two cell-layers, with a mouth and gastro-vascular cavity, and in all cases, ex- 
cept Protohydra, provided with tentacles, which are hollow, forming contin- 
uations of the digestive canal. The body (hydrosome) usually differentiated 
into two sorts of zooids, nutritive (polypites) and reproductive (gonosomes), 
connected by a common stem or nutritive canal (cenosare), the gonosomes 
producing medusa-buds (gonophores), which on being set free are called me. 
duse (or medusoids) and are bisecual.* In these meduse the body is disk 
or bell-shaped, the jelly-like parenchymatous substance composing the disk 
constituting the mesoderm. From the gastro-vascular cavity four primary 
gastro-vascular canals radiate and anastomose with a marginal circular 
canal. No distinct organs of circulation, the blood being sea-water con- 
taining the chyme and a few colorless diood-corpuscles, A true nervous 
system rarely present, but when developed in certain medusoids, forming a 
* Agassiz saw in Rhizogeton, a form allied to Hydractinia, a gonophore which had 
discharged its contents, degenerating into a pulypite or hydra, and its body elongating 
and developing tentacles. Allman observed the same thing in Cordylophora. 
