Gallery 
VIII. 
Table-case 
30 . 
()B CtUIDK to THK T'OSSIL INVERTEUllATK ANIMALS. 
iiiid in them pore.s are clearly seen. I’ores, however, also 
occur in the 1‘ood-^i’ooves of a stalked form, StajanoUastv.s. 
d'he food-grooves of Edriomter (Fig. 31) closely resemble 
those of a starfish, except that tliey, as well as the mouth, 
are protected by covering-plates like those of crinoids. It is 
])ossible that tliese curious lorms may throw some light on 
the origin of starfish. ” 
Since Fdrioasteroidea are rare and of excc])tional interest, 
both British and foreign examples are exhibited together in 
Tal)le-case 30, and are su])pleinented liy rejiroductions. « 
Class ASTEROIDEA. 
On the further side of Table-case 30, tlie free-moving 
Echinodernis begin with the starfishes, generally regarded as 
constituting the simplest Class of Eleutherozoa.' In a starfish 
the liody is either markedly five-sided in outline or is more 
or less star-shaped, in which case it is said to consist of a 
central “ disc ” extended into “ arms,” which vary in number 
from 5 (e.g. the common cross-fish, Asterias) to over 40 (e.g. 
the sun-star, Heliastcr); the moutli is in tlie centre of the 
liody and is turned to the sea-Hoor; the anus is almost in 
the centre of the ujiper surface, but is absent in a few forms ; 
the under side of tlie arms is grooved, and along each groove 
runs a vessel of the hydraulic system ; this vessel gives off 
side-branches which end in free processes (podia) differing 
from those of Pelmatozoa in that each terminates in a 
sucker; between the flooring-plates of the groove ai-e pores, 
through which pass branches from the podia, each com- 
municating with a swelling (amjmlla) within the body. This 
arrangement of the podia enables eacli one to lie extended 
for locomotion, and to be withdrawn into the groove by the 
passage of the fluid from it into the ampulla ; such an 
arrangement is found in no Pelmatozoa excejit perhaps some 
Edrioasteroidea ; but from those forms starfish differ in 
having the groove unprotected by covering-plates. The 
remainder of the starfish skeleton consists usually of small 
plates or bars whicli serve to strengthen and support the 
stout but flexible skin. 
The oldest Palaeozoic starfishes in the British collec- 
tion are Uranaster and I’alacaster, represented liy casts in 
Caradoc sandstone of Upper Ordovician age. From the 
Wenlock lieds comes the heavily plated and many-armed 
Lepidaster. The Lower Ludlow shales of Leintwardine, 
