EcurnoperMAta.| LOWER PALAZOZOIC RADIATA. 51 
a coral greatly larger (or with more interstices in a given space) than the present species, and having on 
the average much fewer cells to the length of a fenestrule; the present species has had the above new name 
applied to it by M. dOrbigny. The magnified figure in the Silurian System has the interstices too 
thick in proportion to the size of the fenestrules. 
Position and Locality.—Rare in the schists of Mathyrafal, S. of Meiford, Montgomeryshire, and 
Llansantfraid, Glyn Ceiriog, Denbighshire. 
6th Class, ECHINODERMATA. 
= Nematoneuwra (Owen.) 
The Echinodermata is the first class in ascending order in which a nervous system has been detected. 
It exists in them in the form of distinct threads (whence the name Nematoneuwra) without - ganglia 
or centres of perception; the most distinct thread round the cesophagus. The Zoophyta have no blood- 
vessels, but a distinct arterial and venous system is present in all the Hehinodermata. Respiration is per- 
formed apparently by the peritoneal surfaces of the interior as well as by the exterior membranes, which 
are covered with cilia to excite the necessary currents; in addition, the Asteriade, Echinidw, Holothurie, 
have pinnate vascular branchize round the mouth, and a peculiar aquiferous system which is apparently the 
most efficient, and is formed by two rows of large vesicles running along the ambulacra, covered with a 
dense vascular plexus of minute blood-vessels, which erate their contents by contact with the sea-water 
with which they are filled; these vesicles give off each two little tubes, which perforate the integument by 
separate openings, and immediately unite to form a long tube, with a terminal disk, to which, from its 
office in locomotion, the term “sucker,” has been applied; these suckers are provided with circular and 
longitudinal muscular fibres, by the contraction of the first of which on the fluid contents of the vesicle the 
tube is extended, and by the action of the others it is retracted and the terminal disk caused to adhere. 
Some of the Echinodermata are soft and smooth, while others are covered with a hard calcareous crust, 
but as they never moult, like crustacea, it is necessarily composed of a number of polygonal pieces, which 
are secreted by the membranes which envelope them and pass between their edges, where the principal 
additions are made as the animal grows larger. Digestion is always performed in a distinctly-walled cavity. 
Previous to the researches of Agassiz, the Hchinodermata were usually considered perfectly radiated 
animals, without a trace of that symmetrical disposition of the parts observable in higher animals; he 
noticed, however, that in Spatangus, there was a flattened ventral disk below, with the mouth near one 
end which was therefore the anterior, the anus at the opposite end defining the posterior extremity, 
and therefore the right and left sides, the general form being elongated in the direction of the length ; 
the whole being perfectly bipartite and symmetrical; of the five ambulacra, one was observed to go directly 
to the mouth, therefore called the anterior, the other two pairs being lateral, leaving one interambulacral 
space (opposite the anterior ambulacrum) at the posterior end, in which the anal opening was situated, 
and which was terminated superiorly by the peculiar large odd, or fifth, porous plate, connected with 
the sand canal, while the four lateral interambulacra were terminated by the ordinary small genital plates ; 
by the coincidence between the position of this fifth plate and the anal opening, whenever the latter 
was excentric, a clue was obtained whereby to recognize the posterior interambulacral space, no matter 
how the position of the alimentary openings was varied—in Ga/erites the mouth was observed to be in 
the middle of the ventral disk, but the anus was still at the posterior margin, and defined the symme- 
trical position of all the rest of the parts; finally, in Hchinus, &c., where the form was spheroidal, without 
perceptible elongation in the direction of the length, and the two alimentary openings were precisely central, 
one in the middle of the ventral aspect, the other vertically over it in the middle of the dorsal, the position 
of the odd porous ovarian plate, determined with certainty as before the posterior interambulacral space, and 
consequently the symmetrical disposition of the remainder of the creature. In the same way the position 
H2 
