3^3 



ECHINOZOA. 



names according to their position and function. In the singular 

 Urchins which constitute the family of the Echinothurida, as also 

 in various Palaeozoic Echinoids, the plates of the test overlap one 

 another in an imbricating manner, so that the shell becomes flexible. 

 As a rule, however, the corona forms an immovable case or box 

 within which the animal is contained ; and its growth is carried on 

 by additions made to the edge of each individual plate by the 

 progressive calcification of an organised membrane which passes 

 between the " sutures," or the lines where the plates come in con- 

 tact with one another. The first circle of the coronal plates is 

 developed round the mouth, and the vertical growth of the test is 

 carried on by the intercalation of successive rows of plates between 

 those already formed and the apical disc. 



The corona is composed of ten alternating meridional zones, of 

 which five are radial in position and are perforated, while five are 

 interradial and are imperforate. In all the recent Echinoids, and 

 in all the fossil forms except the Palechinoids and the Cretaceous 



Tetracidaris, each of these ten 

 a zones is composed of two rows of 



plates, there being thus twenty 

 meridional rows of plates alto- 

 gether. The five interradial zones 

 are spoken of as the " interam- 

 bulacral areas," and are composed 

 of large-sized plates, which are not 



perforated by any apertures (fig. 

 237, ia, and fig. 238, a). The 

 five radial zones, on the other 

 hand, are termed the " ambulacral 

 areas " or " poriferous zones," and 

 are composed of comparatively 

 small plates, which are perforated 

 by minute pores for the emission 

 of the " tube-feet " (fig. 237, a, and 

 fig. 238, b). As a rule, the am- 

 bulacral pores are in pairs, but in 

 a few cases the pores are unpaired. 

 The pores of each pair may be similar or dissimilar in shape, and 

 in many cases they are united by transverse furrows. 



In many Echinoids the ambulacral tube-feet can be protruded 

 along the entire length of the ambulacral areas, which are perforated 

 along their entire course from the centre of the base of the corona to 

 the summit of the same, and which are then said to be " perfect " 

 (ambulacra fierfecta) or "simple" (figs. 237 and 239). In many 

 other Echinoids, on the other hand, the ambulacral areas are not 



Fig. 238. — A, Portion of the test of Holecty- 

 pzis hemisfthericus, enlarged, showing an inter- 

 ambulacral area (a), and an ambulacral area 

 (6). b, Dorso - central system of Hemicidaris 

 intermedia, enlarged : c, Ocular plate ; d, 

 Genital plate ; e, Anal aperture ; /, Madre- 

 porite. (After Forbes.) 



