HYDROZOA CRASPEDOTA. 765 



plus hydranth. There may or may not be a perisarc. Peculiarities are, 

 the transformation of the superficial cells completely or incompletely into 

 protective envelopes in Hydra (p. 327), and the development in Tubular ia 

 of the planula, while within the gonangium, into an Actinula, a hydroid 

 form with a circle of tentacles, or sometimes, as in the adult, two circles, 

 which leads a free existence for a short time before fixation. The planula 

 of Myriothela develops similarly within the vitelline membrane, but the 

 Actinula possesses a set of scattered provisional capitate tentacles in addi- 

 tion to a certain number of permanent tentacles. 



The freshwater Hydroidea are the genera Hydra widely distributed, 

 the American Microhydra from the neighbourhood of Philadelphia, the 

 Russian Polypodium from the Volga (p. 766), and Cordylophora, European, 

 American, and Australian. A Leptomedusan, Laodice (Cosmetira) sali- 

 narum, a modified form of the marine L. (=C.) cruciata, has been found 

 in the variably brackish waters of the discharging canal of saltworks at 

 Villeroy, near Cette, in S. France. Polypodium is in one stage parasitic 

 (p. 766) ; a Leptomedusan Mnestra parasites lives attached by its manu- 

 brium and tentacles to the neck of the pelagic Gastropod Phyllirhoe. 

 The marine Hydroidea are denizens of all seas ; the genera are widely dis- 

 tributed, the species sometimes extremely restricted in their range. They 

 live at very various depths from the surface downwards : the greatest 

 number of forms between the lowest tide-mark and fifty fathoms. Some, 

 however, only occur below one hundred fathoms. At very great depths 

 they appear to be extremely rare. The colonies are as a rule of small or 

 moderate size, but occasionally attain a great height, e. g. a Plumularian in 

 the Pelew Islands that of a man. Deep water forms appear to be larger 

 than those from shallow water. A Monocaulus, seven feet long when fully 

 extended, has been dredged in nearly 3000 fathoms in the North Pacific. 

 The colonies are attached to fixed or floating objects, and certain genera 

 always affect a particular habitat, e.g. Hydr actinia and Podocoryne a Gastro- 

 pod shell tenanted by a Hermit Crab. Some usually grow upon other 

 Hydroids, e.g. some species of Lafoea; Amphibrackium within the canal 

 system of the sponge Euplectella 1 . The Medusae are frequently carried 

 by currents from the localities where their hydroid form grows. A few 

 appear to swim habitually at a considerable depth below the surface. As 

 to fossil forms a Hydractinia with calcareous skeleton is fo.und in the Chalk, 

 and in Tertiary strata (see note I, p. 756, ante). There are some other 

 allied (?) calcareous forms supposed to be Tubularian. The Silurian and 

 Devonian Stromatoporidae perhaps belong to the same sub-order. The 

 Campanularians are doubtfully represented by the Rhabdophora s. Grapto- 

 lithidae, which commence in the Upper Cambrian and die out in the Upper 

 1 Carter has found an undetermined hydroid in the sponge Reniera (A. N. H. (4), x. p. 50). 

 See note 2, p. 781, post. 



