S LADEN : STRUCTURE OF ASTEROIDEA. 
283 
phiura. If this view be correct, the main interest will centre in 
the fact that we have here the representation of a stage in the 
genetic developement of the Asteroidea, all traces of which have 
passed away in the course of the evolution of the more advanced 
forms of the group. 
In conclusion, — the details of structure which have been 
brought forward require but very few words in the way of summing 
up. The facts speak for themselves, and, if we take a common 
sense view of things, the conclusion seems inevitable that the 
homologies now adduced indicate the existence of an ancestral 
relationship between the two groups in question ; nay more, they 
point to a common origin or start-point of descent. From our 
present knowledge it would be difficult, it is true, to demonstrate 
definitely the actual line by which the Asteroidea have descended 
from Ophiuroidea, such as those at present known to us, but I 
submit that this much may be predicated with but little hesitation, 
— that both Asteroidea and Ophiuroidea are the descendents, (along 
collateral lines of development), from a more remote and common 
Ophiuroid-like ancestral stock, but with the actual form of which 
we are at present altogether ignorant, and of the probable structure 
of which we are only able to surmise. 
Bold though such a statement may seem to some, I feel confi- 
dent that biologists will grant that the nature of the evidence is 
such as to remove this conclusion from the region of mere vague 
speculation, and to place it upon the basis of warrantable general- 
isation. 
EXPLANATION OF PLATE XV. 
Fia. 1. — Section through the ray of an Asteroid ( Astropecten), at the 
junction of two segments. Magnified and partly diagrammatic. 
a. Ambulacral plate. 
b. Adambulacral plate. This plate bears the ' ambulacral ' 
spines, which have been omitted from the diagram for 
the sake of clearness. 
e. Supplementary girder-like plate. 
