476 CHARACTERS OF INVERTEBRATE ANIMALS 



other British anemones may be mentioned: Tealia crassicornis, 

 a large red form in which the outside of the body is studded 

 with warts, to which fragments of sand or shells may be found 

 adhering; and the brownish or greenish Opelet (Anthea cereus), 

 which can only partially draw in its tentacles. 



The majority of forms known as "corals" (fig. 291) are also 

 members of the six-rayed order, and they include both simple 

 and colonial species. Of the former may be mentioned a British 

 species (Gary ophy Ilia Smithii) found in the English Channel. 

 In structure it is comparable to a sea -anemone, but a hard 

 calcareous skeleton has been secreted by the ectoderm in the 

 basal part of the animal. When the soft parts are removed 

 this is seen to consist of a conical cup, from which calcareous 

 plates project inward. A simple coral attaining a much larger 

 size is the familiar Mushroom- Coral (Fungia), the flattened cup 

 of which has some resemblance to the top of a mushroom, the 

 gills being represented by the calcareous partitions or septa. 

 Most corals, however, form colonies by budding (gemmation) 

 or splitting (fission), and it is these which chiefly build up the 

 coral-reefs so characteristic of the warmer parts of the ocean 

 where the water is free from sand or mud. All sorts of different 

 shapes may be assumed by the colonies, according to the species, 

 some being encrusting or massive, while others branch in a tree- 

 like way (fig. 291). The individual polypes may be separated 

 by a larger or smaller interspace, so that their separate cups can 

 be clearly made out, either imbedded in or else projecting from a 

 common skeleton secreted by the common or colonial body (cceno- 

 sarc) which unites the different members together. In other cases 

 the polypes are closely packed together, and it even happens 

 in some cases that there is no proper division between them, it 

 being only possible to determine the number of individuals by 

 counting the mouths which are present. 



Order 2. EIGHT-RAYED SEA-FLOWERS (Octactinia) 



Although these forms agree in general plan of structure with 

 corals and sea-anemones they also present striking differences, 

 among which the most obvious are the possession of not more 

 than eight feather-shaped tentacles and the same number of 

 mesenteries. Nearly all the included species are colonial. 



