424 COMPARATIVE ANATOMY CHAP. 



Where only one Polian vesicle occurs, it lies in the left ventral 

 interradius, very seldom in the left dorsal interradius. 



Where two or more vesicles occur, they are also mostly found in the ventral 

 region of the circular canal. 



The walls of the Polian vesicles correspond in structure, essentially, with those of 

 the ring canal. Cells belonging to the inner epithelium become amoeboid and break 

 away from the wall. These are said to become the lymph cells of the water vascular 

 system. 



2. Eehinoidea. In the Spatangoida (which have no masticatory 

 apparatus) the ring canal encircles the oesophagus immediately above 

 the mouth. In other Eehinoidea, however, it is pushed up by the 

 masticatory apparatus which intervenes between it and the mouth. 

 The canal therefore surrounds the oesophagus at the point where this 

 latter emerges from the lantern. The ring canal, as well as its 

 accessory structures, nevertheless, lie within the lantern membrane, 

 which envelops the whole masticatory apparatus. The circular vessel 

 (the lacunar ring) is in close contact with the canal (Fig. 358), 



In the Spatangoida and some Clypeastridce (Echinocyamus pusillus), 

 the ring canal has no accessory structures. In the Stereosomata, on the 

 contrary, it has, in each interradius, a small outgrowth, which ramifies 

 and intertwines with similar ramifications of the circular blood vessel 

 to form together a spongy body, which is known as the Polian vesicle, 

 and is regarded as a lymph gland. This, which, in the Stereosomata, is 

 confined to certain localised interradial points, occurs in the Cidaroida, 

 certain Clypeastroida (e.g. Peronella orbicularis), and the Streptosomata, 

 along the whole course of the canal, so that the intertwining of the 

 appendages of the ring canal and of the circular blood vessel gives 

 rise to a spongy ring. 



An intermediate stage is found in Echinodiscus biforis (Clypeastroid), in which 

 the interradial spongy bodies of the circular canal are longer than in the Stereosomata, 

 but long radially arranged tracts are still left free, the canal at these parts retaining 

 its simple lumen. 



3. Asteroidea. The circular canal which surrounds the mouth, 

 following the inner outline of the oral skeleton, here has two kinds 

 of appendages : Tiedemann's bodies and the Polian vesicles, both 

 of which lie interradially. Tiedemann's bodies appear to occur in all 

 Asteroids, whereas the Polian vesicles are wanting in some families, 

 e.g. the Asteriidce, Echinasteridce, and Linckiidce. 



Tiedemann's bodies (Fig. 363, 7) are small tufts of tubules, closely crowded 

 together, their walls of connective tissue being fused with one another. These 

 tubules, which open into the circular canal, are lined internally with a cubical 

 epithelium, and contain within their lumen bundles of cells which have broken 

 away from the wall. These cells, the protoplasm of which contains pigmented 

 concretions, become the amoeboid lymph cells which float in the fluid of the water 

 vascular system. They give the Tiedemann's bodies their more or less distinct 

 coloration. 



