CLASSi CYSTOIDEA 149 



The Cystideans coiistitute the oldest and least specialised group of the 

 Pelmatozoa. Appearing first in the Cambrian, they develop a great variety of 

 forms in the Ordovician and Silurian, but become extinct before the close of 

 the Carboniferous. While their own ancestry is obscure, it is highly probable 

 that from them have descended both the Crinoids and Blastoids. If, on the 

 one band, the families of Aristocystidae, Sphaeronitidae and Echinosphaeritidae 

 differ radically from Crinoids in respect to their numerously and irregularly 

 plated calyx, or as regards the feeble development or even total absence of 

 their arms ; nevertheless, the Cryptocrinidae and the imperfectly preserved 

 Cambrian genus, Lichenoides, evince a striking similarity, especially as concerns 

 the more or less regulär arrangement of the calyx plates, and a certain 

 approach to radiation. On the other band, forms like Porocrinus and Cleiocrinus, 

 along with strong pentamerous symmetry and regulär arrangement of plates 

 which seem to ally them with the Crinoids, have also, in the presence of 

 pectino-rhombs and calycine pores, characters by which they might with equal 

 propriety be assigned to the Cystids. 



If we can explain the derivation of Crinoids from Cystideans on the 

 supposition that the calyx plates of the latter gradually took on a more 

 definite arrangement, while the loss of pores and pore-rhombs was counter- 

 balanced by a stronger development of the arms and the stem ; so, too, it is 

 possible to derive Blastoids from the same source. Although hydrospires 

 are clearly wanting in the Cystids, nevertheless, other characters, such as the 

 recumbent attitude of the arms upon the sides of the calyx, or their Insertion 

 in grooves on the ventral surface, predicate an intimate relationship with 

 the Blastoids. Probably the most notable similarities are presented by the 

 peculiarly modified families, Callocystidae and Agelacrinidae. Various attempts 

 have been made to affirm a connection between Agelacrinus and the Asteroidea, 

 and between Mesites and certain of the Silurian Echinoidea (Echinocystites) ; 

 but such hypotheses are scarcely warranted, since they proceed from an over- 

 valuation of purely external resemblances, which in nowise prove genetic 

 relationship. 



The Cystids were first recognised as a distinct division of Echinoderms by 

 Leopold von Buch in 1844, but their more detailed Classification long remained 

 in an unsatisfactory condition, and is still involved in considerable difficulty. 

 This is largely owing to the comparative scarcity of material and its 

 frequently imperfect preservation, affording insufficient knowledge of the 

 exact structure in many forms. The Classification of Johannes Müller was 

 based primarily upon the structure of the calyx plates, according to which 

 two main groups were recognised, BJiombifera and Diploporita. To these 

 Roemer afterwards added a third, Aporita, and other divisions were made by 

 Barrande, Neumayr and Steinmann. In later years the Cystids have been 

 treated extensively by Haeckel, Bather, and Jaekel, who have proposed 

 classifications based upon phylogenetic principles. While in some general 

 features these are in substantial agreement they differ considerably in 

 details. The arrangement adopted by Bather, with some of the modifica- 

 tions introduced in the later editions of Zittel's Grundzüge, is in the main 

 here followed. 



