218 ECHINODERMATA—ECHINOIDEA SUB-BRANCH IIT 
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, and 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, Lovén, 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 Regulares (the so-called 
dorso-central plate) was regarded by Lovén 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.! 
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 not 
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 Hchinoidea into two sub-classes—Pale@echinoidea and 
Euechinoidea. 
Sub-Class 1. PALAECHINOIDEA. 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 interambulacrum 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., Calamocrinus Diomedae 
(Mem. Museum Comp. Zool. Cambridge, vol. XVII. pp. 84-96), 1892.—TRans. ] 
