DERMAL SKELETON OF ECHINODEEMA. 205 



plates, whicli lie between the ambulacral tracts (interambulaci'al 

 are^, i), are liomologous with the marginal plates of the arms in 

 the Asteroida. There are two rows of interambulacral, as well as 

 of ambulacral plates. The Echinoida of earlier periods had a larger 

 number of these rows — forms with three, five, and even seven rows 

 in one interambulacral area are known. 



These pieces are connected with one another in different ways. 

 In many Echinoida, as in the Astei-oi'da, the calcareous plates of the 

 perisome are movably connected together, and so allow of changes in 

 the form of the body. In the family of the Echinothurida the 

 plates of the perisome are movably connected with one another, 

 so that the body is able to change its form. The ambulacral as well 

 as the interambulacral plates project over one another like the tiles of 

 a roof in the middle of each area, and the interambulacral ones are 

 separated laterally from one another by a narrow intermediate space. 

 Owing to the thinness of these plates the soft parts of the perisome are 

 of more importance than they are in other families of the Echinoida. 

 These plates are also continued with slight modifications on to the 

 area around the mouth, while in the rest of the Desmosticha this 

 portion is more sharply marked off from the rest. So far the 

 Echinothurida approximate to indifferent conditions, and form an 

 intermediate step between the other Desmosticha, and the hypo- 

 thetical forms which can be derived from the Asteroida. This point 

 of view is further justified by the fact that a firm fascia extends 

 along each ambulacral area on the inner face of the shell, and sepa- 

 rates the parts (nerves, vessels, ampullae) which lie on the ambulacrum 

 from the ccelom. It forms a process, which is attached to either 

 side of an ambulacral groove, and projects some way into the coelom ; 

 this process is, moreover, fenestrated by fine pores. This arrange- 

 ment corresponds to the skeleton of the ambulacral groove in the 

 Asterida, which is calcified in this portion in them ; while the same 

 portion in these forms, in which the perisome corresponding to the 

 ambulacral groove of the Asterida is formed by calcareous plates, 

 remains soft. 



Several important modifications of the regular form of the dermal 

 skeleton of the Echinoi'da, which are not directly comparable with 

 the arrangements found in the Asteroida, are developed ; these are 

 accompanied by the disappearance of the rest of the primitive dorsal 

 perisome, and by the passage of the radiate into other forms. The 

 ambulacral arete no longer extend regularly from the mouth to the 

 backj in the Spatangidge and Clypeastridte they are hmited to a 

 five-leaved rosette (ambulacra petaloidea) placed on the dorsal sur- 

 face. At the same time the number of plates, which is very large 

 in the regular Echinoida, is diminished ; so that there is a smaller 

 number of larger plates. 



The internal skeleton, which in the Asteroida is formed by the 

 skeleton of the ambulacral groove, is represented in the Echinoida by 

 processes of the ambulacral plates. These processes, which are well 

 developed in Cidaris, for example, embrace the nerves as well as the 



