DISTRIBUTION OF HYDROZOA. 71 



found in Northern Europe, being circumiDolar in blieir 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 Lucernariae 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, Placo'is, and Loiocrocis 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 HYDROZOA. 



Body in its simplest form a sac. attached hy tlie aboral end, composed of 

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

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

 tinuations of the hody-cavity. Tlie body (hydrosome) usually differentiated 

 into two sorts of zoaida, nv-tritive ipolypites) and reproductive {gonosomes), 

 connected by a com;mon stem or nutritive canal (cmnosarc), the gonosomes 

 produci)ig medusa-buds {gon/)pliores), which on being set free are called me- 

 dusa (or medusoids) a?id are bisexual.* In these medusm the body is disk 

 or bell-shaped, tlie jelly-like parenchymatous mbstance composing the disk 

 constituting the mesoderm. From the gastro-vascular cavity four primary 

 gastro-vascular canals radiate and anastomose with a marginal cireulai 

 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 when developed in certain medusoids, formi'"^ t 



* Agaesiz saw in Eh'zogeton, a form allied to Hyciractinia, a gonophore which had 

 diticharged its contents, degenerating into a polypite or hydra, and its body elongating 

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



