218 ECHINODERMATA ECHINOIDEA SUB-BRANCH m 



Homologies. The Echinoidea differ radically from the Pelmatozoa and 

 Asterozoa in that arms are completely wanting ; and although the principal 

 viscera (ambulacral, blood, nervous, and digestive systems) are similarly 

 formed and situated, the dermal skeleton, on the other hand, is very differently 

 developed in the three groups. The ambulacra of Echinoids are without 

 arm supports, vertebral disks, or ambulacral ossicles. The system is confined 

 to the interior of the test, arid is covered over with plates which are possibly 

 comparable with the adambulacral plates of the Pelmatozoa and Asteroidea. 

 Great phyletic importance was attached by A. Agassiz, Loven, and P. H. 

 Carpenter to the homology between the dorso-central system of Sea-urchins 

 and the base of Pelmatozoans ; the basals and radials of that system being 

 supposed to correspond to the like-named plates of Crinoids ; and the large 

 central plate developed in the periproct of young Eegulares (the so-called 

 dorso-central plate) was regarded by Loven and others as the solidified homo- 

 logue of five crinoid basals or under-basals, whilst Carpenter considered it to 

 be the equivalent of the radical plate or root-disk at the extremity of the 

 Crinoid stem. Neumayr, Semon, and others have, however, advanced weighty 

 arguments against this hypothesis, and regard all of the above-mentioned 

 resemblances as cases of parallelism. 1 



Habits. Sea-urchins are exclusively marine forms, and all are more or 

 less gregarious. Many species abound in littoral zones, some thriving on the 

 edge of low-water mark (Clypeastridae). Others are found in zones ranging 

 from 400-600 fathoms (Diadematoidea), and a few have been dredged from a 

 depth of nearly 3000 fathoms. Numerous endocyclic Sea-urchins, along 

 coasts exposed to the action of the waves, live in cavities which they bore 

 with their teeth out of the solid rocks. The same species, however, does riot 

 excavate in sheltered places. 



About 300 recent species are known, as compared with fully 2500 

 fossil. The earliest types make their appearance in Ordovician rocks 

 (Bothriocidaris), and continue to be represented sparsely throughout the 

 Palaeozoic era. They multiply enormously in the Mesozoic, and reach the 

 climax of their development in the early Tertiary; after which they begin 

 steadily to decline. As a rule, the species have a very limited vertical range, 

 and hence serve admirably as index fossils. The test is often perfectly pre- 

 served, but even small fragments are capable of accurate determination, owing 

 to the regular repetition of isomeres. 



The number of meridional rows of plates in the corona is used as a 

 basis for dividing the Echinoidea into two sub-classes Palwechinoidea and 

 Euechinoidea. 



Sub-Class 1. PAUEECHINOIDEA. Zittel. 



Test composed of more than twenty meridional rows of plates, or in one order 

 (Bothriocidaroida) of less than twenty. Plates of the areas overlapping or not. 

 Peristome actinal, central, with jaws. Periproct within the dorso-central system or 

 in the posterior inter ambulacrum beyond (Cystocidaroida). 



With the exception of the Triassic Tiarechinus, all members of this sub-class 

 are restricted to the Palaeozoic era. 



1 [For valuable notes on the homologies of Echinoderms, see Agassiz, A., Calamocrinns Diomedae 

 (Mem. Museum Comp. Zool. Cambridge, vol. XVII. pp. 84-96), 1892. TRANS.] 



