212 ECHINODERMATA—ECHINOZOA SUB-BRANCH IIE 
modified so as to be partly branchial in function. Sometimes the tentacles of 
the same ambulacrum differ in shape, structure, and function, in which case 
they are termed heteropodous. Small tufts of external branchiae communicating 
with the ambulacral system are sometimes present, and occupy grooves or 
incisions in the peristomial margin. 
The vascular system consists of a ring-like vascular plexus surrounding the 
oesophagus, and immediately underlying the circular ambulacral vessel. This 
ring gives off five radial vessels, and also two others which send off branches 
to the stomach and generative organs. The central nerve ring, with its five 
principal nerves running down the rays, is external to the two other systems. 
The generative organs are extremely alike in both sexes, and are in the form of 
glands (usually five, sometimes three or even two), situated dorsally and 
interradially on the inner surface of the test. The genital ducts terminate in 
pores in the so-called genital plates, to be described presently. 
Coronal Plates.—The plates of the corona are arranged in ten meridional 
zones. Five of these, the ambulacral areas, are composed of perforated 
plates, and correspond in position to the radiating ambulacral vessels; the 
remaining five, the interambulacral or interradial areas, alternate with the first, 
and are imperforate. 
In all recent, and in the majority of fossil Echinoids, the ambulacral 
areas are composed of two rows of small, alternately arranged plates, the 
inner edges of which meet in zigzag median sutures, and their actinal and 
abactinal edges in horizontal sutures. The interambulacral areas are likewise 
composed of two rows of plates, but they are generally larger than those of 
the ambulacra, and meet them in ambulacro-interradial vertical sutures. There 
are normally, therefore, twenty meridional rows of plates arranged in ten 
alternating zones; but this number is not fully attained in the Palaeozoic 
Bothriocidaroida, and is exceeded in the remainder of the Paleechinoidea, in the 
Triassic Tiarechinus, and in the Cretaceous Tetracidaris. (The number of plates 
is the same in all of the ambulacral and all of the interambulacral areas 
respectively ; but the two systems are entirely independent of one another as 
respects the size, shape, and number of the plates. In the Cidaridae, for 
example, the ambulacra are very narrow, and are composed of from fifty to 
sixty plates in each column ; the interambulacra are much broader, and con- 
sist of four or five large plates in each column. In the regular Sea-urchins, 
or Endocyclica, all of the ambulacra and all of the interambulacra are similar ; 
but in the Hzocyclica, the anterior ambulacrum and the posterior interambu- 
lacrum often differ considerably from the corresponding areas. 
Interambulacral (interradial) plates are always simple ; ambulacral plates 
may be either simple or compound. In the latter case, they may be formed 
of two or of several components, all of which are joined by sutures and form 
a more or less geometrical plate. Most simple plates, and some of the 
components are primaries—that is, they extend from the outer edge of an 
ambulacrum to the median suture of the area. Demi-plates is a name applied 
to those components which do not reach the median line. 
The growth of the test in all Echinoids is effected by new plates being 
successively added at the aboral termination of the ambulacra and inter- 
ambulacra, and by their increasing in size and solidity. In the young 
condition generally, and in the Clypeastroids and Spatangoids throughout 
life, the interambulacra begin ventrally with a single plate, situated outside the 
