THE ECHINOIDEA 



293 



folded branchiae or gills (Fig. VII. b) lying on the margins of the 

 peristomial membrane. These are diverticula from the general 

 body-cavity, and pass out by the ten notches in the peristomial 

 margin of the test. There are, moreover, five large internal 

 vesicles rising from the upper edge of the masticatory apparatus ; 

 these are known as "Stewart's organs" (Fig. VII. st), and may 

 act as internal gills. 



The ordinary Echinus, then, has the following characters : It 

 consists of a skeleton, which is mostly external, and is com- 

 posed of numerous closely fitting polygonal plates, bearing spines. 

 Within, it has a simple, coiled, alimentary canal, with mouth and 

 anus at the opposite poles ; it has five generative glands ; an 

 elaborate series of water-vascular vessels, provided with podia, 

 ending in suckers. This water-vascular system, the blood-vascular 

 system, and the nervous system each consist essentially of a 

 ring round the mouth, from which five branches pass outward, 

 one up the inside of each ambulacrum. 



Among the Echinoids the Variations in Structure from this 

 simple type are very diverse. Thus the form, instead of being 

 globular, may be depressed into a thin, flat sheet, in which the 

 wide, low roof has to be supported by pillars, as in Scutella (Fig. 

 XXXV. 4). In some of these thin forms the posterior margin of 

 the test is lobed and digitate, as in 

 Rotula (Fig. IX.) ; in some cases the 

 ends of the processes unite, leaving 

 perforations or " lunules." In such 

 depressed forms, owing to the sharp 

 division of the test into upper and 

 lower halves, the central podia of 

 the former are useless for purposes 

 of locomotion, and are specialised 

 to serve as branchiae ; thus the am- 

 bulacra become modified into petaloid 

 and extra - petaloid portions. The 

 ejection of the excreta through an 

 anus situated in the middle of these 

 branchiae would be disadvantageous, 

 owing to the consequent pollution of 

 the water. Hence, in such forms, the anus has passed backwards, 

 and opens in the hinder part of the test. This backward move- 

 ment of the anus is usually balanced by the forward movement 

 of the mouth, and thus the Echinoid loses its quinqueradiate 

 symmetry and becomes bilaterally symmetrical. 



This change affects not only the position of the external 

 apertures, but the development of the internal organs. Owing to 

 the invasion of the posterior interradius by the anus, the generative 



Fio. IX. 



Rotiila Avgusti, with posterior "digita- 

 ions " and a pair of anterior " lunules." 



