745 



CLASS HYDROZOA. 



Marine, rarely freshwater, Coelenterata; free or fixed; simple or colonial. 

 There are two forms of zooids, the Hydroid and the Medusa, the former 

 asexual except in Hydra, the latter alone, or one of its degenerate forms, 

 sexual. Both forms of zooids usually possess tentacles : the mouth is circular 

 or square : the gastric cavity either simple, or provided with four ridges, or 

 in the Medusa partly obliterated, leaving pouches or canals radiating from a 

 central cavity. Sensory cells and ganglion cells may be found in the Hydroid ; 

 aggregated sense-cells, ocelli and auditory organs, nerve rings or ganglionic 

 centres in the Medusa. The generative organs are either ecto- or endo-dermic. 

 The sexual zooid is developed from the asexual, either directly by metamor- 

 phosis, or indirectly by gemmation or fission, thus giving rise to an Alternation 

 of Generations. 



There are two principal types of the Hydroid. One, the Hydromedusan 

 or Craspedote type, consists typically of an oral and stomachal region 

 (hydrocephalis), with or without tentacles, borne upon a peduncle (hydro- 

 cope). The tentacles are variable in shape and disposition, rarely tubular, 

 usually solid. The mouth is placed at the extremity of an oral cone, and 

 is sometimes extremely dilatable ; the gastric cavity is simple, but its endo- 

 derm cells are sometimes thrown into ridges by the contraction of the 

 musculature. The second or Acrasped type, the Scyphostoma, has a 

 squarish mouth in the centre of a peristome, which is fringed by a circle of 

 solid tentacles ; a somewhat flask-shaped body and a short peduncle. Its 

 gastric cavity is traversed by four equidistal longitudinal ridges, into which 

 the mesoglaea enters. The Hydroid is (i) a permanent locomotor sexual 

 form, multiplying by gemmation, but only temporarily colonial, Hydra : 

 (2) a larval form which passes by a metamorphosis into a Medusa, which 

 may multiply by gemmation but is only temporarily colonial, its hydriform 

 progeny alone in some instances attaining the Medusa-stage, Trachyme- 

 dusae, Acrasped Pelagia-. (3) A non-sexual but permanent form, sometimes 

 solitary, usually however multiplying by gemmation, which is rarely dis- 

 continuous, but as a rule continuous, giving origin to colonies, most Hy- 

 droidea, Acraspeda : (4) a locomotor sexual form, probably derived by the 

 specialisation of (3), and never multiplying by gemmation Acrasped De- 

 pas tridae and Lucernaridae. 



The Medusa 1 is a bell or disc-shaped organism with the mouth at the 

 apex of a manubrium, which depends from the centre of the concavity of 

 the bell or disc. The mesoglaea of the aboral aspect of the bell is much 

 thickened to form the umbrella s. exumbrella, that of the oral remains thin, 

 and is known as subumbrella. The rim of the bell may carry hollow or 



1 See p. 247, ante, and Fig. n. 



