ASTERIAS PALLIDA. 275 



iiumicate with each other but not with their fellows on the other side. 

 Tlie pair of perihœmal spaces lying nearest the centre of tlie disc 

 grow around the water-vascular ring- canal in opposite directions 

 away from the median mesentery, leaving this untouched 1)ut co;des- 

 cing with the cavi(y of tlie next radius when the two have «-rown 

 enough to meet each other, and thus form the periliiemal ring spaces. 

 In the accompanying cut continuous lines represent tliese spaces before 



and dotted lines the condition after, 



^~~4%, /9à' fusion. In this way is brouuht 



'"^^'--'■'--'-^-^^ about the condition of the peri- 



ff \\ ha?mal system tliat we see in the 



Jj rt adult starfish, the central jxjrtion 



"-^'^^^ ,.</^-^-^---^f ^^' ^vhich is represented diagram- 



matically, together with the 



qti circidar enterocœl and the central 



ciid .-PI 



ii'! portion ot the water-vascular 



Cut 4. system, in Fig. 49. 



The correctness of mv oijinion above explained as to the origin of 

 the perihajmal spaces is still more strengthened, if ptxssible, by the con- 

 dition that we meet with at the tip of the arins of a growing startisli. 

 In Fig. o4 is represented a, small ])ortion next the terminal tentacle of 

 a radial section of an arm of a young star, whose radius was 5 mm. 

 Here we see that the perihremal system consists of a number of entirely 

 separate cavities, each lying between the roots of the tube-feet, although 

 in the more central portion of the section the ])erihiemal space consists 

 of a continuous cavity on either side of the radial plane. Hence we 

 may infer that the peripheral portions of the periliœmal system of the 

 young starfish grow by a, repetition of just the same processes as those 

 by which it was formed at its very beginning, viz., by the formation 

 of separate mesenchymatous cavities and their subsequent fusion. 



