vill ECHINODERMATA — WATER VASCULAR SYSTEM 425 



Two Tiedemann's bodies usually occur in each interradius ; the intenadius 

 containing the stone canal not infrequently, however, forms an exception to this 

 rule, only one such vesicle being present in it {Asleriidic, EehinastericUe, Linckiidii:, 

 Asterinida:, Culcitidcc). If the circular canal is viewed internally in the position 

 shown in Fig. 363, this body lies to the right of the stone canal. 



The Polian vesicles (Fig. 363, 6) are large structures with long stalks, and to 

 them, as to Tiedemann's bodies, the function of lymph glands has been attributed. 

 In the Asterinida; C'ulcitida; Lnidia, and several species of Astropedcii , one 



Fio. 3G3.— Circular canal, Polian vesicles, Tiedemann's Ijodies, and ampullae of tlie water 

 vascular system of Asterina glbbosa (after Cu^uot). Seen from within, i.e. from the body cavity. 

 1, Mouth at the centre of the oral membrane ; 2, stone canal ; 3, axial sinus ; 4, transverse muscles 

 of the ambulacral plates ; 5, ambulacral plates ; C>, Polian vesicles ; 7, Tiedemann's bodies ; 8, circular 

 canal ; 9, blood vascular ring(?) ; 10, ampulla. 



vesicle is found in each interradius. Only in tlie interradius containing the stone 

 canal is it wanting, or else (in species of Astro2iecten) two are here found instead of 

 one. Astropecten aurantiacus has two to four (usually three) Polian vesicles in each 

 interradius (even in the stone canal interradius). The wall of this vesicle, proceeding 

 from without inwards, consists of: (1) the ciliated endothelial covering ; (2) a layer 

 of connective tissue in which run the longitudinal muscle fibres ; (3) a circular 

 muscle layer, and (4) the inner epithelium, whose cells lie in the interstices of a net- 

 work of connective tissue. 



4. Ophiuroidea. — The water vascular ring here possesses one 

 Polian vesicle, which functions as lymph gland in each interradius 

 except that of the stone canal. The structure of the wall of this 

 vesicle resembles that in the Asteroklea, the longitudinal musculature, 

 however, seems always, and the circular musculature frequently, to be 

 wanting. The canals to the first two tube-feet arise directly from the 

 circular canal, commencing usually as a common canal which forks 

 later, but occasionally the canals are distinct from the first. 



Ophiactis mrens (Fig. 364) occujnes an exceptional position among the Ophiuroidea, 

 being capable of asexual reproduction by means of fission. This form not only has, 

 as already mentioned, several stone canals, but in each interradius two to three 



