HYDROZOA. 



53 



Fig. 15.1 



Case 4. 



efficiency even on old dried specimens. The degenerate dactylozooids Case 4. 

 have a number of capitate tentacles, while the gastrozooids have only 

 a whorl of four (Fig. 10). 



In 1891 Prof. Hickson discovered the male, and in 1898 the 

 female Medusa (Fig. 17) of Millepora in small capsules, which, when 

 occurring near the surface, form rounded swellings (ampullae). 

 Latterly, Mr. Duerden has seen the living Medusae in his aquarium 

 at Jamaica. This tiny Medusa is only about J^ inch in diameter ; 

 its cavity is nearly filled up by the large manubrium containing the 

 eggs. The umbrella is devoid of canals, tentacles, and sense-organs, 

 but is provided with batteries of thread- 

 cells ; and usually no mouth can be 

 seen at the end of the manubrium. 

 The little creature, however, is able to 

 swim away with its heavy burden of 

 eggs from the parent colony ; having 

 deposited the eggs, it shrivels up. 



Stylasterid^. In this family the 

 dactylozooids are without tentacles, and 

 one or both kinds of zooids are supported 

 in their calicles by a calcareous style. 

 There are several genera in this family. 



In Stylaster the pores are arranged 

 in " cyclo-systems " — a circle of dacty- 

 lopores surrounding a central gastropore. 

 A cyclo-system presents a deceptive 

 resemblance to the calicle of an ordinary 



coral ; in the latter the calicle contains one coral polyp, but in the 

 cyclo-system there are a dozen or more degenerate individuals sur- 

 rounding a central individual ; the dactylozooids were formerly 

 supposed to be the tentacles of the central zooid. The generative 

 buds, which are situated in the often numerous swellings or ampullae, 

 never become free Medusa3. 



The Stylasters are remarkable for the elegance and beauty of 

 their arborescent fan-shaped foims (Fig. 14) and their exquisite 

 colouring. Several specimens of Stylaster roseus from off a cable 

 from the West Indies show considerable variation in colour, being 

 white, rose-pink, and salmon-coloured. The cyclo-systems regularly 

 alternate on the sides of the slender branches. 



Fragment of Millepora, show- 

 ing the eirdea of dactylopores 

 each with a central gastropore. 

 (Twice natural size.) (After 

 Moseley.) 



From " Encyclopsediai Britannica,' 



