Blastoidea. — Ccelentera. 103 



They had short stems and a large number of fine, short, Blastoidea 

 unbranched arms, arranged on the sides of five broad grooves (see GALLERY 

 figure 157 on p. 101). Folded pouches, communicating with the j.^J"^/, 

 outer water, were developed inside the cup, and probably assisted 

 respiration, whence they are called *'hydrospires." The British 

 Blastoids (Table-case 75) and the foreiga ones (Wall-case 18) 

 contain most of the specimens described by Etheridge and Carpenter 

 in their monograph, published by the Trustees. 



GALLERY X. 



IV.— CGELENTEEA. . 



This large group formed the greater part of Cuvier's division Ccelentera. 



** Eadiata," which included also the Echinoderma and some less- GALLEEY 



. .X. 



known groups. It is now, however, known that the radiate -^est Side. 



symmetry upon which this old division was founded is of less wall- 



systematic value than the internal anatomy. All those forms, cases 1-6, 



such as the Echinoderms and the Bryozoa, which have the cases 1-10, 



digestive tube definitely separated from the general body-cavity, 



are removed from their alliance with those in which there is no 



such definite separation. The latter group forms the phylum 



of the Ccelentera. This is subdivided into two classes, the 



Hydrozoa and the Actinozoa, by the same character. The latter 



includes those which, like the sea-anemones, have a partially 



developed digestive tube, which is, however, open freely below 



to the general body-cavity, and tlms is " ccelenterate " instead of 



*' coelomate." The class Hydrozoa is the simpler, and has no such 



special digestive tube, the inner lining of the body-cavity serving 



also for the preparation and absorption of the food. The three 



typical animals belonging to the Ccelentera are the fresh-water 



Polype or Hydra, the Jelly-fish, and the Sea- Anemone. These 



are all soft-bodied animals, and it is therefore not surprising that 



several orders are either unknown or very rarely found in the 



fossil state. The simplest type of skeleton consists only of 



spicules: these fuse together into a spicular skeleton, as in the 



red coral. In others carbonate of lime is deposited in the 



tissues, and a hard, platy, calcareous skeleton is then formed. 



The last condition is met with in ordinary corals: as in them 



