280 
S LADEN : STRUCTURE OF ASTEROIDEA. 
traces have entirely disappeared long before the adult stage is 
arrived at. 
It will be unnecessary for me to point out to those acquainted 
with the general principles of Biology, the significance and extreme 
importance of this fact ; and it will suffice to say for the present 
occasion, that this devolves upon a law in embryology — that the 
phases in the individual development of a complex organism re- 
present, in a more or less epitomised manner, the conspicuous char- 
acters of the stages passed through during the course of phylogenetic 
development, or in other words, symbolise the race-history of the 
animals to which they belong. 
From the above-mentioned circumstance we reason that there 
was a time in the history of Asteroids when under arm -plates were 
possessed, and when in fact these Echinoderms were conformed to 
that plan of arrangement which we now associate with the 
Ophiuroidea only. 
The homologies of parts just enumerated may be further pur- 
sued if we study the comparative anatomy of the mouth-armature 
of the two groups, and this investigation will at the same time 
furnish a proof of the correctness of the foregoing assertions. Such 
a circumstance is not difficult to understand when it is borne in 
mind that th? mouth-parts are formed by the concrescence and 
modification of certain of the ray elements ; and secondly, that 
representative elements, or in other words, homologous parts go 
to the formation of similar portions of the dental apparatus in both 
groups. As the discussion of this question in detail would 
absorb far more space than the present occasion would allow, I 
shall not extend my remarks further in that direction, having 
simply mentioned the fact as an evidence of the close affinity that 
may be traced between these apparently widely separated forms. 
There is yet another point in the structures of the Asteroidea 
which ought not to be passed over when critically discussing the 
race-history of the group ; and it is also an eiabryological observa- 
tion for which we are indebted to Mr. Alex. Agassiz* ; although I 
* Proceed. Am Acad. Arts and Sci., April, 1863. 
