212 



PHYLUM ECHINODERMATA. 



side the apical system. The ambulacral plates so formed are biporous 

 and of similar size and shape. In many forms they retain these characters, 

 and the ambulacral plates are simple and biporous. In other cases they 

 soon become unequal in size and fused together to form compound plates, 

 in which there are two, three or more pairs of pores. The growth of the 

 ambulacra and interambulacra are quite independent of one another. 

 According to Loven the peristomial plates of the Cidaridae are detached 

 successively from the corona and are formed in the same manner, but 

 earlier, as are the other plates of the corona. 



The plates of the ambulacrum next the peristome are called 

 the marginal ambulacral plates. Apically an ambulacrum ends 

 in one of the ocular (radial) plates. 



f In the interambulacra the plates are not 



perforate nor composite. Apically an inter- 

 ambulacrum abuts upon a genital (basal) 

 plate, and the plates next the peristome 

 are called the marginal interambulacral 

 plates. The peristomial margin is often 

 incised, i.e. there is a notch between the 

 two peristomial plates of each interam- 

 bulacrum ; these notches are for the passage 

 of the external gills (see p. 231). 



The peristomial membrane always con- 

 tains a number of calcareous bodies, but 

 these in the Spatangoids and Clypeastroids 

 are not perforated and have nothing to do 

 with the ambulacral and interambulacral 

 series of plates. In the Cidaroida, however, 

 both the ambulacral and interambulacral 

 plates are continued on to the peristome, 

 and are flexibly joined together by mem- 

 brane. In the Diadematoida it contains (except in the Echino- 

 thuridae, in which the ambulacral series of plates is continued 

 up to the mouth) five pairs of perforated ambulacral plates 

 (Fig. 148), which carry the buccal tentacles (see p. 233). 



In the irregular forms or Exocyclica (Holectypoida, Clypeas- 

 troida, Spatangoida) the anus and periproct are not contained 

 within the apical system which keeps its position at the upper 

 pole, but lie at some point in the posterior interradius, either 

 on the upper surface of the shell or at the ambitus or on the 

 lower surface (Figs. 152, 153). The Holectypoida alone retain 

 the spherical and radiately symmetrical form. In the other 



Flo. 151. Third ambula- 

 crum of a young Strongy- 

 locentrotus droebachiensis 

 of 3 mm. (from Claus, 

 after Loven). Op radial 

 plate ; P primary plate 

 and double pore ; Cp 

 tubercle for spine. The 

 ambulacral plates are 

 compound and the sut- 

 ures are visible. 



