2 1 2 ECHINODERMATA ECHINOZOA SUB-BRANCH in 



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 Palceechinoidea, 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 interarnbulacra 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 interarnbulacra are similar ; 

 but in the Exocydica, 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 



