258 PALEONTOLOGY 



These are usually three in number, but may be two or 

 four. Thus there is nothing in Echinosphcvra showing 

 pentamerous symmetry, unless it be the anal pyramid. 

 Certain other cystids, however, give a hint of how the 

 pentamerous condition arose : they have also three food- 

 grooves, but while the anterior one (opposite the anus) 

 remains unbranched, the other two branches soon divide 

 into two, giving five grooves with an imperfect radial 

 symmetry (Fig. 73, c). 



The Cystidea are divided into four orders : 



1. Amphoridea, with no trace of radial symmetry, 

 varying from simple sacs like Aristocystis (Ord. of 

 Bohemia) to forms with well-developed stem, flattened 

 theca and more or less bilateral symmetry. Dendrocystis 

 (Fig. 73, a, Ord.) has one long arm, with the mouth at 

 its base, the anus at the opposite end near the stem, both 

 on the same side, while on the other side are outgrowths 

 (ABr and A A) which served to balance them. There 

 are still more specialized bilateral forms like Placocystis 

 (Sil.) with a curious mimicry of Branchiopod Crustaceans, 

 which suggests that they were actively moving animals. 



2. Rhombifera, showing the rhomb - foldings of 

 stereom described above. Echinosphceva is about the 

 simplest of these. Others like Macrocystella (Tremadoc) 

 show stem, circlets of thecal plates, and arms almost like 

 crinoids. Others like Lepadocrinus (Fig. 73, b, Sil.) show 

 a remarkable specialization of a few (usually three) of 

 the rhombs into large pectinirhombs with all the appear- 

 ance of respiratory organs (gills), while the food-grooves 

 are on the crests of ridges which might be described as 

 arms sessile on the theca. 



3. Aporita, a small group with fair pentamerous 

 symmetry and no special stereom-structures, e.g. Crypto- 

 crinus (Ord.) 



