A proposed division of the Phylum Echinodermata

Vol. XXII, ^pp. 183-184 October _30, 1909

PROCEEDINGS

^'

OF THE

BIOLOGICAL SOCIETY OF WASHINGTON

A PROPOSED DIVISION OF THE PHYLUM ECHINO- DERMATA.

BY AUSTIN HOBART CLARK.

Many

attempts

have

been

made

toelucidate the interrelations between the various

echinoderm

_classes, but

none

of the _pro- posed

arrangements have

^{been able to stand thetest}of critical investigation. ^I

have

_recently

shown

that addition to the ambulacral post-radial ^series of ossicles in the crinoids takes place

by

the interpolation of ossicles between the first

two

(the first

two

brachials of the free

arm) and

the radials

("

_interpo-

lated division series"); each ^{division series of}

two

ossicles represents

/Wr

^{ambulacral plates as}

^found

^{in the urchins;}

^two

fuse,

forming an

axillary,

and

onedisappears, ^its

proximal and

distalarticularfacesbeing therebyprojected

upon

^eachother,^fus- ing,

and producing

^the

non-muscular

articulationsby

which

the

components

^{ofdivision series of}

two

ossicles(or^of the

two

_pairs in cases

where

the division seriesareoffourossicles)are united.

The method

^ofambulacral increaseis, therefore, similar in the crinoids

and

echinoids. In addition, the crinoids

add segments

monoserially ^at the

extreme

_{tip of}their long

and

slenderarms.

It has been supposed^{that this increase}

was comparable

^{to that}

ofophiuroids

and

_asteroids, butin _reality it is

merely

^{a distant} analogy, ^{for in} the asteroids

and

_ophiuroidsthe plates are

added

biserially at the

end

ofthe

arm,

^but just

proximal

^{to a}

perma- nent

terminal _plate.

The

ossicles

up

^to

and

_including the sec-

ond

brachial of the undivided crinoid

arm

are _strictly

homolo-

gous ^{tothe entire}ambulacral seriesin

an

urchin; the

remainder

ofthe free

arm

represents theauricles ofthe urchin.

A

^critical

comparison

^ofthe

anatomy

^ofurchins

^and

^crinoids

^shows them

to be closely related,

and

_very different in almost every

way

29—Pkoc. Biol. Soc.Wash., Vol. XXII, 1909. (183)

184 Clark

—

_Proposed Division _ofthe

Phylum

Echinodermata^.

from

the ophiuroids

and

asteroids,

which

are also similarly closely^related.

The

holothurians belong with ^theformer group.

Detailed anatomical evidence of this will shortly be published in the

American

Naturalist.

The

_following is proposed ^{as the}

arrangement

^best

showing

the interrelations of the classes in the

phylum Echinodermata

in the _{light of}the

most

recent knowledge:

PHYLUM ECHINODERMATA.

I.

Sub-phylum Echinodermata

Hp:teroradiata.

1. Class Pel matozoa.

a. Sub-class Crinoidea.

b. Sub-class Cystoidea.

c. Sub-class Blastoidea.

2. Class Erhinoidea.

3. Class Holothuroidea iBohadschoidca).

n. Sub-phylum Echtnodehmata Astroradiata.

1. Class Ophivroidea.

2. Class Asteroidea.