278 COELENTERATA HYDROZOA chap. 



Fam. Aequoreidae. — In this family the hydrosome stage is 

 not known except in the genus Polycanna, in which it resembles 

 a Campanulariid. The sense-organs of the Medusae are statocysts. 

 The radial canals are very numerous, and the genital glands are 

 in the form of ropes of cells extending along the whole of their 

 oral surfaces. Aequorea is a fairly common genus, with a 

 flattened umbrella and a very rudimentary manubrium, which 

 may attain a size of 40 mm. in diameter. 



Fam. Thaumantiidae. — The Medusae of this family are dis- 

 tinguished from the Aequoreidae by having marginal ocelli in 

 place of statocysts. The hydrosome of Thaumantias alone is 

 known, and this is very similar to an Obelia. 



Fam. Cannotidae. — The hydrosome is quite unknown. The 

 Medusae are ocellate, but the radial canals, instead of being 

 undivided, as in the Thaumantiidae, are four in number, and very 

 much ramified before reaching the ring canal. The tentacles are 

 very numerous. In the genus Polyorchis, from the Pacific coast 

 of North America, the four . radial canals give rise to numerous 

 lateral short blind branches, and have therefore a remarkable 

 pinnate appearance. 



Fam. Sertulariidae. — In this family the hydrothecae are 

 sessile, and arranged bilaterally on the stem and branches. The 

 general form of the colony is pinnate, the branches being usually 

 on opposite sides of the main stem. The gonophores are adelo- 

 codonic. Sertularia forms more or less arborescent colonies, 

 springing from a creeping stolon attached to stones and shells. 

 There are many species, several of which are very common upon 

 the British coast. Many specimens are torn from their attach- 

 ments by storms or by the trawls of fishermen and cast up on 

 the sand or beach with other zoophytes. The popular name for 

 one of the commonest species (S. ahietina) is the " sea-fir." The 

 genus has a wide geographical and bathymetrical range. Another 

 common British species frequently thrown up by the tide in great 

 quantities is Hydrallmania falcata. It has slender spirally- 

 twisted stems and branches, and the hydrothecae are arranged 

 unilaterally. 



The genus Grummaria, sometimes placed in a separate family, 

 is distinguished from Sertularia by several characters. The stem 

 and branches are composed of a number of tubes which are con- 

 siderably compressed: The genus is confined to the southern seas. 



