SLADEN : STRUCTURE OP ASTEROIDEA, 
275 
ON TRACES OF ANCESTRAL RELATIONS IN THE STRUCTURE OF 
THE ASTEROIDEA. BY W. PERCY SLADEN, F.L.S., F.G.S. 
[Plate XV.] 
The Echinodermata constitute a sub-kingdom of animals in which 
the structure of the various divisions presents a number of 
highly remarkable points of affinity and difference. Indeed, so 
strikingly is this the case, that when typical or representative 
specimens are subjected for the first time to superficial examination 
by a person unacquainted with their internal anatomy, the princi- 
pal groups would most probably be looked upon by him as perhaps 
the most diverse of creatures — popularly speaking — that it could 
be possible to link together. Five Classes of living Echinoderms 
are included in this category : — 
1 . The Crinoidca, which are familiarly known as ' Sea-Lilies. ' 
2. The Ophiuroidea or ' Brittle -Stars.' 
3. The Asteroidea or 'Starfishes.' 
4. The Ecliinoidea or ' Sea-Urchins. ' 
5. The Holothuroidea or 1 Sea-Cucumbers.' 
The present communication will deal with certain homologies 
that exist in the structure of the Asteroidea and Ophiuroidea, upon 
which deductions of considerable importance have been based. It 
will be desirable, however, to explain before proceeding further 
that although the word ' homologies ' is employed in this instance 
as synonymous with ' traces of descent,' it is not the purpose of this 
paper to trench upon the subject of the phylogenetic development 
of the Echinodermata as a whole, but simply to confine the remarks 
to several points of evidence that indicate ancestral relations be- 
tween the two groups in question. 
Although we shall not directly press into our service palseont- 
ological evidence, I make bold to believe that the subject will not 
be without interest to Geologists ; — first, from the circumstance that 
Asteroids and Ophiuroids are amongst the earliest representatives 
of the Echinodermata with which we are acquainted; and secondly, 
because the facts about to be enumerated encroach upon one of 
