CNIDARIA 133 



Narcomedusae, and in the Siphonophora remarkable colonies are 

 found which appear to have originated by budding from a medusa. 



The following orders are contained in the class : 



Calyptoblastea (Leptomedusae) . Hydrozoa in which the coenosarc 

 is covered by a horny perisarc, produced over the nutritive polyps as 

 hydrothecae and over the reproductive individuals as gonothecae; 

 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 Qgg shell. 



Trachylina. Hydrozoa without "alternation of generations", the 

 medusoid developing directly from the egg; with tentaculocysts, and 

 with generative organs lying on the radial canals or on the floor of the 

 gastric cavity. 



Hydrocorallinae. Hydrozoa existing as sessile colonies with an 

 external 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 of anthomedusan type. 



The Graptolitoidea (see p. 148) 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. 112) root-like hollow tubes (the 

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

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

 giving off 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 in the axils of the hydranths, 

 polyps modified for reproduction, the blastostyles, occur. The whole 

 system of tubes which connect up the individual polyps is the coeno- 

 sarc, and it must be understood that the enteron or cavity of the 



