DISTRIBUTION OF IIYDROZOA. 71 



found in Northern Europe, being cireumpolar 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 Lucernarise are arctic as well as temper- 

 ate forms, while Cyanea is peculiar to the Northern Hemi- 

 sphere. Aurelia saad. Pelagia are cosmopolites, while Rhaco- 

 pilus, Placo'ls, 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- 

 dr 'actinia 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. 



CLASS I. THE IIYDROZOA. 



Body in its simplest form a sac attached by theaboral end, composed of 

 two cell-layers, with a mouth and gastro-vascular cavity, and in all cases, 

 except Protohydra, provided icith tentacles, which are hollow, forming con- 

 tinuations of the body -cavity. The bodi/ (liydrosome) usually differentiated 

 into two sorts of zooids, nutritive (polypites) and reproductive (gonosomes), 

 connected by a common stem or nutritive canal (cauwsarc}, the yonosomes 

 producing medusa-buds (gonophores), which on being set free are catted me- 

 dusfe (or medusoids) and are bisexual.* In these medusce the body is disk 

 or bell-shaped, the jelly-like parenchyniatous substance composing the disk 

 constituting the mcsodenn. 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 blood-corpuscles. A true nervous 

 system rarely present, but lohen developed in certain medusoids, 



* Agassiz saw in Rhizogeton, a form allied to Hydractinia, a gonophoro which had 

 discharged its contents, degenerating into apulypite or hydra, and its body elongating 

 and developing tentacles. Allmau observed the same thing in Cordylophora. 



