CLASS I ECHINOIDEA Hila 
plates, covered with soft structures, and carrying spines. Mouth on the under or 
actinal surface ; anus situated either in the dorso-central system or somewhere in the 
posterior interambulacrum. Plates of the five ambulacral areas more or less ex- 
tensively perforated for the protrusion of tube feet. 
The Test—The test or covering of Echinoids is composed of numerous 
calcareous plates, firmly united by their edges, so as to form a more or less 
rigid case or box, and disposed in certain regions or systems. In a few 
instances, however, the plates overlap one another in an imbricating manner, 
so as to impart a certain degree of flexibility to the test. The main element 
of the test is called the corona, and the system of plates at the summit is 
known as the dorso-central or apical system. 
The test is pierced by two large openings, which are surrounded by 
membranes studded with small calcareous particles. One of these is the 
mouth, which is always placed on the under or actinal surface of the body, 
may be either central or eccentric in position, and is surrounded by a cori- 
aceous peristomial membrane. The other aperture is the anus, surrounded by 
a periproctal membrane. In some orders it is placed opposite the mouth in 
the centre of the apical system, in which case the test is said to be regular or 
endocyclic. But in other orders it is situated beyond the apical system at a 
variable distance in the median line of the posterior interambulacrum, either 
dorsally or actinally ; the test is then said to be irregular or eaocyclic. 
The mouth opens into an oesophagus, which conducts into a capacious 
stomach, and thence into a convoluted intestine. The digestive tract winds 
round the interior of the test, being attached to the inner surface of the 
latter by muscles, and terminates in the anus. Surrounding the oesophagus 
is a circular vessel filled with water, which is admitted by the so-called 
stone canal, opening externally in a madreporite. This is a porous or sieve-like 
structure, consisting of a variable number of canals, and though commonly 
restricted to one of the plates of the apical system, it may extend backwards 
and perforate a large part of the system. 
The circular vessel gives off five branches, known as the radiating canals, 
which pass along the ambulacral areas on the interior of the test, and connected 
with it in the interambulacral areas are five distensible membraneous reservoirs, 
termed the Polian vesicles. The radiating canals give off numerous lateral 
branches or tube feet (tentacles, pedicels) which are capable of protrusion through 
pores of the ambulacral plates. Emission is effected by means of secondary 
vesicles or ampullae which by contraction force their contained fluid into the 
tube feet and distend them. The ampullae, as a rule, communicate with the 
tube feet by two canals perforating the plates separately, a single tentacle 
being placed over a pair of ambulacral pores. The tube feet serve normally 
as locomotive organs, when they are prehensile and end in a suctorial disk ; 
but in many forms, especially those having petaloid ambulacra, they are 
XIV.), 1882-84.—Schliiter, C., Die reguliren Echiniden der norddeutschen Kreide (Abhandl. zur 
geolog. Special-Karte von Preussen, Bd. IV.), 1883.—Jdem, Neue Folge, Heft 5, 1892.—de Loriol, 
P., Description des Echinides (Commission des travaux géol. du Portugal, vol. II.), 1887-88.— 
Lovén, S., On Pourtalesia (Svensk. Vetensk. Akad. Handl., Bd. XIX.), 1884.—Pomel, NV. A., Class- 
ification méthodique et genera des Echinides vivantes et fossiles, 1883.—Duncan, P. M., Revision 
of the Genera and Great Groups of the Echinoidea (Journ. Linn. Soc., vol. XXIII.), 1889.—Zbert, 
T., Die Echiniden des nord- und mittel-deutschen Oligocins (Abhandl. zur geol. Spec. Karte von 
Preussen, Bd. X.), 1889.—Zovén, S., Echinologica (Bihang till Svensk. Vetensk. Akad. Handl., 
Bd. XVIII.), 1892.—Clark, W. B., Mesozoic Echinodermata of the United States (Bull. U.S. Geol. 
Survey, No. 97), 1893. 
