102 



Guide to the Invertehrata. 



Cystidea. 

 GALLERY 



vni. 



East Side. 

 "Wall-case 

 18, Table 

 case 75. 



YI. — Cystidea. 

 The Cystids (Sack-forms) are the oldest Echinoderms that 

 we know, and approach more nearly to the parent stock from 

 which all the classes have been derived. Owing to their great 

 diversity of form and structure, it is difficult to make any general 

 statements concerning the Cystids as a class. Some have slits in 

 the test, by which, it is probable, the internal fluid was aerated 

 (e.g. Lepadocrmus). Some have pores; others have neither slits 

 nor pores. Some have arms free, and either 2, 3, 4, 5, or 6. 

 Others have pinnules fringing ambulacra on the surface of the 

 test (Lepadocrinus). Some are spherical (e.g. Sphceronis) ; others 

 are flat and oblong (e.g. Placocystis). They were once so numerous 

 as to form rock-masses, as shown in some specimens of Sphceronis 

 and Echinosphcera, the Crystal- apple of the Scandinavians (Wall- 

 case 18). The British collection (Table-case 75) is very complete. 



Fig. 1. 



Fio. 158. — Placocystis Forbesianus, De Koninck, a flattened Cystid. "Wenlock 

 Limestone : Dudley. 

 Convex side, showing the facets for the spines {t, t), and the point of 

 attachment for the stem («). 

 S. Concave side, showing portions of the spines {t, t). 

 3. View of lower extremity of the body, showing attachment of stem (s). 

 h. View of the top of the body, showing the facets for the spines {t, t). 

 5. Portion of stem near the body : drawn from a specimen having a 

 portion of the stem still remaining attached. 

 I, b, c. Three views of a small tapering stem, found detached, but 

 ornamented with wavy ridges like those on the body-plates. 

 Probably the lower extremity of the stem. 

 One of the spines: drawn from a specimen having the spine still 

 attached to the body. The apparent joints are only cracks. 

 The originals of these drawings are in Table-case 75. 



<5. fl. 



Tal)Ie-ca8e 

 75, Wall- 

 caae 18. 



YII. — Blastoidea. 

 The Blastoids (Bud-forms) were a curious little branch from 

 the Cystid stock dominant in Devonian and Carboniferous times. 



