112 Guide to the Invertehrata. 



GALLERY forms a series of canals radiates from the body-cavity), and in 



^" which the reproductive organs are contained in external buds. 



e^libie!^ The animals may be simple, as in the fresh-water Polype 



cases {Ilydra)^ or compound, either growing in plant-like tufts, as 



® * ^®' the Sea-Fir {Sertularia), or massive corals, as in Millepora. The 



colonies are usually fixed, as in the two examples quoted, but in 



some they are free, as in the order Siphonophora, such as the 



Portuguese Man -of- War. In many other forms the reproductive 



individuals ("gonophores") are free-swimming, though the normal 



forms are fixed. 



Of the five orders of the Hydrozoa, four contain only soft-bodied 

 animals, and are not represented in the collection ; but in one, the 

 Hydroidea, many genera have horny or calcareous skeletons, which 

 may be preserved in the fossil state. The Hydroidea are divided 

 into the following groups : — 



1. — Corynida, in which the polypes are usually tubular, and pro- 

 tected by a calcareous or chitinous covering, but without 

 cup-shaped expansions. 



2. — Hydrocorallina, in which the polypes are elongated and 

 tubular, and protected by a massive, calcareous skeleton. 



3. — Graptolitoidea, an extinct group in which the skeleton was 

 usually strengthened by a rod. 



4. — Thecaphora, forms in which the polypites are protected by 

 cups or hydrothecae. 



1. The Corynida are not a group of much geological importance. 

 The two most interesting forms included within it are Hydr actinia 

 and Parheria, shown in Table-case 9. The former is represented 

 by some specimens encrusting shells from the Crags, and the latter 

 by some spherical fossils from the Cambridge Greenland, which 

 were formerly regarded as Foraminifera. 



2. The Hydrocorallina are mostly included in the same case. 

 In the Chalk there is the genus Forosphcera, which has been regarded 

 by some authorities as a sponge. The most important fossils, how- 

 ever, in this group belong to the family Stromatoporidae, of which' 

 a large series is shown in polished slabs of the Devonian Marbles. 

 The genus Stromatopora also occurs in the Silurian rocks, where 

 it is associated with lahechia, both of which are shown in Table- 

 case 10. 



