COELENTERA TA 



57 



of lime are built up, simulating some of the true corals. 

 Certain tubular spaces are left for the accommodation of the 

 hydranths, which can be withdrawn into them. The hydranths 

 are colonial, and of two kinds : gastrozoids, provided with mouths, 

 which are the nutritive persons of the colony ; and dactylozoids, 

 elongated mouthless persons well armed with nematocysts. 



The order is divided into two famihes : (i.) the Mille- 

 PORIDAE and (ii.) the Stylasteeidae. 



(i.) The Millepoeidae. — This family may have an arborescent 

 or branching skeleton, consisting of an outer living part 

 which surrounds an inner and older dead 'part. The 

 younger parts of the fores, in which the living hydranths 

 are found, are separated from the older ones, wherein 

 their predecessors lived, by horizontal partitions known as 

 tabulae. Hie colonies are hermaphrodite. 



(ii.) The Stylasteeidae. — These are always arborescent, never 

 encrusting, and their colonies are unisexual. 



The hydranths may be scattered irregularly, but more 

 usually the gastrozoids are surrounded by a regular circle of 



Fig. 41. — Longitudinal median section 

 througli a Stylasterid. After Hiclcson. 



1. Gastrozoid. 



2. Dactylozoid. 



3. Style. 



i. Coenosarcal canals. 



5. Coenosarc, calcareous. 



6. Mouth. 



7. Tentacle. 



dactylozoids. The gastrozoid (Fig. 41) is usually provided 

 with tentacles, but in some of the Stylasteeidae these are 

 wanting. The dactylozoids are long tentacular structures with 



