746 THE ANIMAL KINGDOM. 



solid tentacles, and the mouth may have solid marginal tentacles. The 

 gastric cavity in the bell is generally partially obliterated. Organs of 

 special sense are generally present. Sexual organs are developed on the 

 walls of the manubrium, gastric cavity, or radial canals, from the ectoderm, 

 or in the gastric cavity from the endoderm. Asexual reproduction by 

 gemmation, very rarely by fission, occurs in some Craspedota. The Medusa 

 swims by alternate contractions and expansions of the bell. It is derived 

 (i) by direct metamorphosis from a larval Hydroid, Tr achy medusa^ 

 Acrasped Pelagia: (2) by gemmation from a hydroid colony, Hydroidea: 

 (3) by the multiple transverse fission of a Hydroid, most Acraspeda^ (4) by 

 gemmation from a Medusa, some Craspedota. 



The hydroid of the Craspedota may become polymorphic (pp. 757-8), 

 and the Medusa be degenerate (pp. 762, 768) or devoid of mouth, and 

 simply a locomotor or hydrostatic organism (pp. 771-2). 



In one order, the Siphonophora, precocious gemmation takes place in 

 the embryo to form colonies containing polymorphic hydroid individuals, 

 and generally medusoid as well. The Siphonophora are consequently to be 

 regarded as the most specialised group in the class. 



Skeletal structures in the Hydrozoa are confined to the Hydroid, and 

 are secreted by the ectoderm. They occur in two forms, most commonly 

 as a chitinoid investment, the perisarc, more rarely as a calcareous coeno- 

 steum. The axial endoderm cells of the solid tentacles have a tough cell 

 membrane and vacuolated contents. 



The sexes are, as a rule, separate in the individual. Segmentation is 

 total, and generally equal : the larva a free swimming planula 1 . 



The Hydrozoa are widely distributed and almost exclusively marine ; 

 a few are fresh-water, but some of the marine forms are tolerant of brackish 

 or even of fresh- water (p. 748). They are essentially carnivorous. Some of 

 the Campanularian Hydroidea, some Craspedote and Acrasped Medusae, 

 and especially among the latter the genus Pelagia, some Siphonophora, e.g. 

 Diphyes^ Abyla> &c., are phosphorescent. Fossil forms are rare. Two 

 extinct groups, the Silurian Graptolithidae, and the Silurian and Devonian 

 Stromatoporidae, are generally considered as Hydroids. A few Medusae 

 have been described from the Jurassic Solenhofen slates, and one from the 

 Chalk. One or two of them appear to belong to the Tr achy medusae, the 

 remainder to the Acraspeda. 



There are two sub-classes, the Craspedota and Acraspeda. 



1 Metschnikoff has classified the formation of the endoderm in this class as follows : (A) Multi- 

 polar, and then (i) by primary delamination, i. e. transverse fission of. the cells of the single layered 

 embryo (Geryonidae, Eudendrium) ; (2) by immigration of cells on all sides (Aeginopsis) ; (3) by 

 secondary delamination, i. e. arrangement of the cells (Aglaura, Rhopalonema, most degenerate 

 Hydroidean Medusae) ; (4) by a combination of the foregoing (Polyxenia leucostyld). (B) hypo- 

 tropic, and then by (i) immigration at one pole (Hydroidean Medusae) ; (2) by invagination (many 

 Acraspeda}. See Embryol. Studien an Medusen, Wien, 1886, pp. 70, 71. 



