﻿OF THE ACALEPHiE OF NORTH AMERICA. 325 



view is natural, it will probably be found that the vibrating locomotive fringes of Pleu- 

 robrachia are, among the many complications of animal structure, precisely that step 

 in their development where the complication of the isolated cilia has reached its ex- 

 treme, and has been made the foundation of a higher stage of development, in which 

 the parts, which, in the primitive cell, were simply structural complications, assume an 

 independent existence, developed by the growth of new cells. 



Fully to understand the character of the vertical rows of locomotive fringes, it 

 should be borne in mind that they are connected for their whole length with vascular 

 tubes following the same course (Plate III. Fig. 1 to 4), and which arise from the great 

 central chymiferous cavity. This intimate connection leads naturally to the suppo- 

 sition that, besides their functions as locomotive organs, the vertical rows of fringes are 

 in some way connected with respiratory functions, and that, there is between these 

 two systems the same natural physiological connection which exists in Echinoderms 

 between the inner branchiae and the ambulacral tubes ; or in Worms, between the 

 respiratory vesicles and the locomotive bristles. 



The circulation of fluids, and the respiratory movements connected with this cir- 

 culation, are almost throughout the animal kingdom in direct communication with loco- 

 motion, even in the higher animals. Among Polypi the dilatations and contractions of 

 the body renew constantly the water which fills their cavity, and provide them with 

 a fresh supply of aerated water. The same is the case among Medusa?. For even 

 where there is no distinct individualized system of respiratory organs, it is obvious 

 that a constant renewal of the surrounding medium, by means of which oxygenation 

 takes place, is an essential condition for the maintenance of life ; and where there are 

 no special organs adapted to this purpose, the main movements of the body supply the 

 deficiency. The water-pores in Echinoderms, through which their main cavity is con- 

 stantly filled with fresh sea-water, undoubtedly perform a similar office. Again, among 

 Mollusca, respiration and locomotion are still more intimately connected ; but in a man- 

 ner which differs decidedly from what we observe in higher animals. For there, by 

 the dilatation and contraction of the respiratory cavities and the circulation of the 

 blood through the respiratory organs, the body is amply supplied. But unless Acephala 

 open their valves, unless they expand and contract alternately the whole body, the supply 

 of fresh aerated water must be much less ; and I doubt whether oysters and clams could 

 be kept alive if their valves were shut constantly by pressure, and muscular motion, 

 contraction and expansion of the large bundles which preside chiefly over locomotion, 

 were prevented from coming into play in aid of the vibratory cilia of the mantle and 

 gills. The manner in which the respiratory cavity is shut in so many Gasteropods, 



