HYDROZOA 165 



ponds and canals in England. It has a polyp -like stage, Mtcrohydra, 

 which has a certain likeness to Hydra. 



Order HYDROCORALLINAE 



The forms included in this group are mostly associated with reef 

 corals in tropical seas. The main part of the colony consists of a much 

 branched hydro rhiza with frequent anastomoses. Instead of secreting 

 a horny perisarc as the Calyptoblastea and the Gymnoblastea do, the 

 ectoderm lays down an exoskeleton consisting of calcareous grains, 

 which becomes bulky and solid. It may be either massive or encrust- 

 ing or branching. From pits in the surface of the colony arise the 



AacL 



tab. 



Fig. 123. Diagrammatic section through Millepora showing a gastrozooid 

 with a dactylozooid (dact.) on each side of it and an ampulla (amp.) with a 

 medusa enclosed in it; can.i, the living canals, shown in black, and can.2, the 

 degenerating canals, shown as lines, constitute the hydrorhiza, and the 

 skeleton is represented by stippling ; med. a medusa just liberated ; tab. tabulae 

 in a gastropore. Slightly altered from Hickson. 



polyps. These are of two types (Fig. 123). First there are the in- 

 dividuals of normal structure with a mouth surrounded by tentacles 

 {gastrozooids) : these nourish the colony. Then there are the dactylo- 

 zooids which are much longer and more slender. They have no mouth 

 but they possess scattered capitate tentacles and may form a ring 

 round a gastrozooid, in which case it is readily observed that their 

 function is to catch prey and hand it to the central gastrozooid for 

 digestion. Besides the polyps there are the medusae, which, as in 

 Bougainvillea, are budded directly off from the coenosarc: they are 

 lodged in pits of the skeleton called ampullae^ but their liberation has 

 been observed in Millepora. It is supposed, however, that their free- 

 living existence is very brief. 



