226 MINERALS AND GEOLOGY 



A. Without natatory cilia : 



A. 1 Stomach-cavity completely identical with body-cavity 

 (i.) Without internal calcareous corallum : 



Class I. Hydrozoa. 

 (ii.) With internal calcareous corallum : 



Class II. Hydrocoralla. 



A. 2 Stomach partially separated from body-cavity : 

 (i.) Mouth-opening with eight fringed tentacles : 



Class III. Crossocoralla or Alcyonaria. 

 (ii.) Mouth-opening with numerous simple tentacles . 

 Class IY. Anthocoralla or Zoantharia. 



B. With natatory cilia : 



Class V. Ctenophora.* 



HYDROZOA. 



This class, as here defined, is composed of soft-bodied aquatic types, 

 without internal stony " corallum. "f In some of its representatives, 

 however, a chitonous or horny, cellular support is present. The 

 class may be sub-divided broadly into the following orders : 1. 

 Hydrida (e.g., the fresh-water Hydra); 2. Hydromedusce (e.g., 

 Tubularia, Sertulaiia, &c., and extinct Graptolites) ; 3. Discomedusce 

 (e.g., the true Medusce, JKMzostoma, <fec.); 4. Lucernarida (e.g., 

 Lucernaria, &c.) ; and 5. Siphonophora (e.g., Physalia, Velella, <fec.). 

 Of these, the Hydrida, Lucernarida, and Siphonophora, have no 

 fossil representatives. The Discomedusce, being entirely soft-bodied, 

 gelatinous types, are most rare in the fossil state ; but their impres- 

 sions have been occasionally found in Mesozoic rocks, as in the 

 lithographic slates of Solenhofen in Bavaria. The remaining Order, 

 that of the Hydromedusce, represented by the living sertularians, &c., 

 contains, on the other hand, an extinct group of forms, the Grapto- 

 lites, of great palseontological interest. These forms are exclusively 

 of lower palaeozoic age, and are typically characteristic of Silurian 

 strata. 



Graptolites : The extinct forms, thus known, were apparently 



* An aberrant group, forming a passage-group into the echinodermata. Fossil representa- 

 tives are unknown. 



t The Milleporidce are Hydrozoids with secreted calcareous corallum, and should thus h 

 placed with the Hydrocoralla, as in the present classification. See page 229. 



