MILLEPORINA 259 



an adequate account of the general anatomy. The colony 

 consists of two kinds of zooids — the short, thick gastrozooids (Fig. 

 129, G*) provided with a mouth and digestive endoderm, and 

 the longer and more slender mouthless dactylozooids {D) — united 

 together by a network of canals running in the porous channels 

 of the superficial layer of the corallum. The living tissues of 

 the zooids extend down the poi?e-tubes as far as the first tabulae, 

 and below this level the canal-system is degenerate and function- 

 less. It is only a very thin superficial stratum of the coral, 

 therefore, that contains living tissues. 



The zooids of Millepora are very contractile, and can be with- 

 drawn below the general surface of the coral into the shelter of 

 the pore-tubes. When a specimen is examined in its natural 

 position on the reef, the zooids are usually found to be thus con- 

 tracted; but several observers have seen the zooids expanded in 

 the living condition. It is probable that, as is the case with 

 other corals, the expansion occurs principally during the night. 



The colony is provided with two kinds of nematocysts — the 

 small kind and the large. In some colonies they are powerful 

 enough to penetrate the human skin, and Millepora has there- 

 fore received locally the name of "stinging coral." On each of 

 the dactylozooids there are six or seven short capitate tentacles 

 (Fig. 129, t), each head being packed with nematocysts of the small 

 kind; similar batteries of these nematocysts are found in the 

 four short capitate tentacles of the gastrozooids. The nemato- 

 cysts of the larger kind are found in the superficial ectoderm, 

 some distributed irregularly on the surface, others in clusters 

 round the pores. The small nematocysts are about 0-013 mm. 

 in length before they are exploded, and exhibit four spines at 

 the base of the thread; the large kind are oval in outline, 

 0-02 X 0-025 mm. in size, and exhibit no spines at the base, but 

 a spiral band of minute spines in the middle of the filament. 

 There is some reason to believe that the filament of the large 

 kind of nematocysts can be retracted.^ 



At certain seasons the colonies of Millepora produce a great 

 number of male or female Medusae. The genus is probably 

 dioecious, no instances of hermaphrodite colonies having yet been 

 found. Each Medusa is formed in a cavity situated above the 

 last-formed tabula in a pore-tube, and this cavity, the " ampulla," 

 1 S. J. Hickson, Willey's Zool. Results, pt. ii. 1899, p. 127. 



