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 “ papule” (abactinal or dorsal water- 
tubes, A. Agassiz; Hautkiemen, Ludwig; respiratory processes, Carpenter; Kiemen- 
blischen, Hamann ; tubules, Voot and Yung; branchies lymphatiques, Cuénot; dermal 
branchiz, Durham), which puncture the body-wall in the form of delicate transparent mem- 
branous coeca, 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,” 
they would also seem to permit of the passage of “scavenging amceboid cells” and more 
or less solid particles. The papule 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 papulee 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 papule 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 recard 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 papule are present; those first formed are confined 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 papule 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. 261. 
2 Proc. Roy. Soc. 1888, vol. xliii. p. 329. 
