xxiv THE VOYAGE OF H.M.S. CHALLENGER. 



acters whose differences indicate changes in structural detail resulting from modification 

 and development, and show a relationship of subordination which warrant their being 

 ranked as ordinal factors : — 



1. The adaptation of the organism to subserve the functions of respiration and 



excretion. 



2. The character of the ambulacral skeleton. 



3. The character of the ambital skeleton. 



I will remark briefly on each of these topics, confining, however, my observations 

 on the present occasion to the sub-class Euasteroidea, as the other constituents of the 

 Asteroidea are fossil forms whose classification it is not my intention to discuss in this 

 place. 



1. The organs which Stimpson ' first named " papulae " (abactinal or dorsal water- 

 tubes, A. Agassiz ; Hautkiemeu, Ludwig ; respiratory processes, Carpenter ; Kiemen- 

 blaschen, Hamann ; tubules, Vogt and Yung ; brauchies lymphatiques, Cudnot ; dermal 

 branchiae, Durham), which puncture the body-wall in the form of delicate transparent mem- 

 branous cceca, permit an exchange by osmosis of fresh oxygenated fluid from without, and 

 of the effete or carbonised fluid from within the body-cavity. According to Mr. Durham, 2 

 they would also seem to permit of the passage of "scavenging amoeboid cells" and more 

 or less solid particles. The papulae may be distributed over the whole body, or may be 

 confined to a limited area. By means of their mode of occurrence, the Euasteroidea may 

 be divided into two groups : in one the papulae are confined to the abactinal surface, and 

 never pass beyond the boundary of the supero-marginal plates, and consequently do not 

 occur in the lateral walls or on the actinal surface ; in the other group the papulae extend 

 beyond the boundary of the supero-marginal plates, and occur in the lateral walls and on 

 the actinal surface. The former of these groups may be called the Stenopneusia, the latter 

 the Adetopneusia. I regard the first group (the Stenopneusia) as the older, and as indi- 

 cating a simpler or less complex stage of organisation for the performance of the functions 

 in question. Embryology supports this view, for at an early stage in the life history of 

 an Adetopneusate Asterid no papulae are present; those first formed areconfined to the 

 abactinal surface, and the earliest to appear are situated near the base of the ray. It is 

 only at a later stage of growth that the papulae invade the lateral walls and the actinal 

 surface. In other words, the members of the more highly developed group (the Adeto- 

 pneusia) pass in the course of their development through a stage which represents the 

 characters of the adult condition of the more primitive group (the Stenopneusia). 



2. The ambulacral skeleton — by which I understand not only the ambulacral plates 

 and their associated adambulacral plates, but also the correlated series of tube-feet — exhibits, 



1 Proc Boston Soc. Nat. Hist., vol. viii. (1862), p. 2G1. 



2 Proa. Roy. Soc, 1888, vol. xliii. p. 329. 



