420 COMPARATIVE ANATOMY CHAP. 



yet known, there is always only one stone canal, and it always com- 

 municates with the exterior by means of the pores of the madreporite. 

 This communication is, however, by no means direct. The pores of 

 the madreporite first lead into a small cavity lying below it, the 

 madreporie ampulla, into which opens, on the one hand, the ascend- 

 ing stone canal, and, on the other hand, the axial sinus of the entero- 

 coel, to be described later. The stone canal, on leaving the ampulla, 

 traverses the body cavity, following the axial sinus Avith its lymph 

 gland, and runs down to the water vascular ring, which in the 

 Cidaroida and Clypeastroida encircles the oesophagus immediately above 

 the masticatory framework (Fig. 358), but, in the Spatangoidct, imme- 

 diately above the mouth. In both the former groups the stone canal 

 is short and more or less straight, but in the Spatangoida it is very 

 long and runs in coils. 



On the possibly great morphological importance of the ampullae, cf. the section 

 on Ontogeny. 



Echinocyamus pusillus, a Clypeastrid, shows an embryonic condition in the 

 adult in that the madreporite has only one pore. All other Echinoids, examined 

 with reference to this point, possess as adults several or numerous pores. The 

 number of pores increases with age and growth. 



The pore canals which traverse the madreporites may anastomose with one 

 another. They may enter the ampulla through several inner pores or else through 

 one common inner aperture. In the Spatangidcc, they traverse the substance of a 

 large skeletal process (apophysis) of the madreporite, which projects into the cavity 

 of the test. 



The condition of the stone canal in the Spatangoida deserves further investiga- 

 tion, since the observations hitherto recorded contradict one another. According to 

 one account, the stone canal (in EcMnocardium) breaks up into branches on its way to 

 the water vascular ring, these branches communicating with the axial blood lacuuar 

 system. According to another, it ends blindly (in Spatangus purpurcus), and the 

 water vascular ring is said in no way to communicate openly with the apical stone 

 canal. A canal, however, runs from the water vascular ring towards the stone canal, 

 without reaching it. The existence of any kind of communication with the lacunar 

 system is emphatically denied by those who hold this latter view. 



3. Asteroidea. In all Asteroids the madreporite is external, 

 and takes the form of a skeletal plate, which is perforated by many 

 pores, and always lies on the apical side of the disc interradially. 

 The stone canal, within the axial sinus and attached by a band to its 

 wall, descends direct to the water vascular ring which surrounds the 

 oesophagus, and enters this ring interradially. The wall of the stone 

 canal is generally highly calcined, and its lumen is divided in a more 

 or less complicated manner into shelves, niches, etc., by projecting 

 folds which frequently branch. It not infrequently happens in 

 Asteroids that there is more than one stone canal and madreporie 

 plate. For example, all Asteroids which reproduce asexually (i.e. by 

 division) possess more than one canal. 



The relations of the madreporite to the axial sinus are interesting. 

 Not all the pores of the madreporie plate open into the stone canal ; 



