OF THE ACALEFHiE OF NORTH AMERICA. 359 



fissure which separates the two large lobes rises higher along the sides of the mouth, 

 and thus introduces a loop in the lateral ambulacra, instead of a straight course, as on 

 the sides ; the vibratory cilia of the small lobes being modifications of the locomotive 

 combs of the ambulacra proper, which would appear as long on this side as on the 

 other, if they were stretched in the same manner, but which are here folded over in the 

 shape of prominent auricles acting more directly and energetically as lateral oars. 



If this view of the four small lobes is correct, we may consider the vertical branch 

 or fork of the vascular tube below as the direct prolongation of the ambulacral tube 

 proper, and the fork which diverges into the large lobes as the anastomotic fork, con- 

 necting the ambulacral tubes all round the body. The horizontal branches along the 

 sides of the mouth should then be considered as the anastomotic branches between the 

 two lateral ambulacral tubes of each side, and thus the circle would be made perfect. 



There is a great interest connected with a further investigation of the vibratory 

 cilia of the small lobes, in comparison with the locomotive combs of the ambulacra. 

 For the former are so similar to. common vibratory cilia, and the latter are so plainly a 

 more advanced system of locomotive apparatus with a complicated structure, and both 

 are morphologically so fully homologous, as to show that, in the series of natural develop- 

 ments, vibratory cilia are not absolutely a specific type of structure, but may constitute, in 

 a gradual development, a natural link, connecting more complicated organs with the sim- 

 plest fringes of structural cells. I entertain now so little doubt respecting such transitions, 

 that 1 have not hesitated throughout these descriptions to consider the rows of vertical 

 locomotive fringes as true ambulacra, though there is as great a difference between them 

 and the ambulacra of Echinoderms, as there is between them and simple vibratory cilia. 

 We are thus led to recognize through the whole type of Radiata a natural gradation in 

 the structure of the organs through which currents of water are produced around the 

 body, from the simplest combinations in Polypi to the most complicated apparatus in 

 Echinoderms. In Polypi we have only vibratory cilia arising from structural cells over 

 extensive surfaces of the whole body, while in Beroid Medusae there are, in addition 

 to such cilia, peculiar rows of fringes, which move by muscular action upon their bases, 

 and in Echinoderms each fringe in the shape of an independent ambulacral type as- 

 sumes as great a structural complication as the whole system taken together in Aca- 

 lephae. The ambulacral tubes in Echinoderms, and the aquiferous system with its 

 vesicles in star-fishes, or the true ambulacral gills in Echini, seem to me, indeed, to 

 bear the same relation to each other, as the fringes of the locomotive combs, with 

 their basal muscles, in Beroid Medusae, bear to the vertical ambulacral tubes. 



If, from this review of the superficial ramifications of the chymiferous tubes, we 



