154 THE INVERTEBRATA 



the medusae flattened, with gonads on the radial canals, and usually 

 statocysts. 



Gymnoblastea (Anthomedusae). Hydrozoa in which the coenosarc 

 is covered by a horny perisarc which stops short at the base of the 

 polyps and reproductive individuals; the medusae bud-shaped, the 

 depth of the bell greater than the width, with gonads on the manu- 

 brium and eyes, but not statocysts. 



Hydrida. Hydrozoa existing as solitary polyps without medusoid 

 stage; tentacles hollow; without perisarc, the polyps being capable of 

 locomotion; gastrula forms a resting stage encased in an egg shell. 



Trachylina. Hydrozoa in which the medusoid is large and the 

 hydroid phase minute. The latter either forms medusa buds or being 

 represented by the planula larva metamorphoses into a medusa. 

 Statocysts with endodermal concretions : generative organs lying on 

 the radial canals or on the floor of the gastric cavity. 



Hydrocorallinae. Hydrozoa existing as fixed colonies with an ex- 

 ternal calcareous skeleton into which the usually dimorphic polyps 

 can be retracted. 



Siphonophora. Hydrozoa existing as free-swimming, polymorphic 

 colonies, without perisarc, derived by budding from an original 

 medusiform individual. 



The Graptolithina (see p. 169) are probably another order of the 

 Hydrozoa and certainly belong to the class. 



Orders CALYPTOBLASTEA, GYMNOBLASTEA, 



HYDRIDA 



We will take as examples of these orders Obelia, belonging to the 

 Calyptoblastea and Bougainvillea to the Gymnoblastea, both of which 

 produce free-swimming medusae, and then describe Tubularia with 

 its sessile gonophores. The series ends with Hydra (Hydrida). 



In a colony of Obelia (Fig. 114) root-like hollow tubes (the hydro- 

 rhiza) run over the surface of attachment, such as a seaweed, and 

 from these spring free stems, which branch in a cymose fashion 

 giving ojff the polyp heads (hydranths) on alternate sides. At the 

 growing ends of the main branches are produced buds which develop 

 into hydranths, and towards the base of the branches in the axils of 

 the hydranths, polyps modified for reproduction, the blastostyleSy 

 occur. The whole system of tubes which connect up the individual 

 polyps is the coenosarc, and it must be understood that the enteron 

 or cavity of the colony is continuous and common to all its members. 

 The rhythmical contraction of the hydranths causes currents which 

 distribute the food obtained by some individuals to those parts of the 

 colony where feeding is not taking place. As in all Calyptoblastea the 



