2 84 COELENTERATA — HYDROZOA chap. 



pores and dactylopores are not provided with tabulae except in 

 the genera Pliobotlirus and Sporadopora. The character which 

 gives the order its name is a conical, sometimes torch-like pro- 

 jection at the base of the gastropore, called the "style," which 

 carries a fold of the ectoderm and endoderm layers of the body- 

 wall, and may serve to increase the absorptive surface of the 

 digesti^'e cavity. In some genera a style is also present in the 

 dactylopore, in which case it serves as an additional surface for 

 the attachment of the retractor muscles. The pores are scattered 

 on all aspects of the coral in the genera Sporadopora, Errina, and 

 Plioiothrus ; in Spinipora and Steganopora the scattered dactylo- 

 pores are situated at the extremities of tubular spines which 

 project from the general surface of the coral, the gastropores 

 being situated irregularly between the spines. In Phalangopora 

 the pores are arranged in regular longitudinal lines, and in 

 Bistichopora they are mainly in rows on the edges of the 

 flattened branches, a single row of gastropores being flanked by 

 a single row of dactylopores on each side. In the remaining 

 genera the pores are arranged in definite cycles, which are fre- 

 quently separated from one another by considerable intervals, and 

 have, particularly in the dried skeleton, a certain resemblance to 

 the calices of some of the Zoantharian corals. 



In Cryptohelia the cycles are covered by a lid-like projection 

 from the neighbouring coenenchym (Fig. 1 3 6, Z 1, Z 2). The gastro- 

 zooids are short, and are usually provided with a variable number of 

 small capitate tentacles. The dactylozooids are filiform and devoid 

 of tentacles, the endoderm of their axes being solid and scalariform. 



The gonophores of the Stylasterina are situated in large oval 

 or spherical cavities called the ampullae, and their presence can 

 generally be detected by the dome-fshaped projections they form 

 on the surface of the coral. The female gonophore consists of a 

 saucer-shaped pad of folded endoderm called the " trophodisc," 

 which serves the purpose of nourishing the single large yolk- 

 laden egg it bears ; and a thin enveloping membrane composed of 

 at least two layers of cells. The egg is fertilised while it is 

 still within the ampulla, and does not escape to the exterior 

 until it has reached the stage of a solid ciliated larva. All the 

 Stylasterina are therefore viviparous. The male gonophore has 

 a very much smaller trophodisc, which is sometimes (Allopora) 

 prolonged into a columnar process or spadix, penetrating the 



