BLASTOIDS. 
3°3 
Devonian of the same country. Parasites, however, find crinoids an easy and 
almost unresisting prey. A suctorial crustacean, eggs and all, has been found in 
the body - cavity, 
while a decapod 
crustacean occasion¬ 
ally inhabits the in¬ 
testinal tube. The 
annexed figures re¬ 
present the cysts 
formed by the crinoid 
in response to the 
irritation set up by 
the presence of a 
parasitic worm, in 
which cysts it takes 
up its abode. There 
are also worms that 
bore into the stem, as 
well as boring sponges, and corals that affix themselves to the stem. The crinoid 
generally makes some attempt to overwhelm these intruders by the rapid deposition 
of the calcareous skeletal substance; so that in the rocks greatly thickened stem- 
fragments are found enclosing the remains of corals, brachiopods, etc. 
The Blastoids, —Class Blastoidea. 
The Blastoidea constitute a compact group, pretty clearly marked off from 
both Cystidea and Crinoidea, which they resemble in the upward position of the 
mouth and the generally fixed habit. The chief character that separates blastoids 
from other echinoderms is the presence of an elongate plate, the lancet-plate, 
underlying the ambulacrum and pierced by a canal supposed to have contained the 
radial water-vessel. These five canals meet in a circular canal round the mouth, 
but there is no evidence that they were connected with tube-feet as in other 
echinoderms. Each side of each ambulacrum was lined by a row of delicate, 
unbranched arms; and the food-grooves of these arms passed to a single groove 
running down the middle of the surface of the ambulacrum, and these five grooves 
then passed up to the mouth. 
The most interesting structures in the Blastoidea are the hydrospires. In such 
a form as Pentremites there are five openings (spiracles) round the mouth, placed 
in the interradial areas between the ambulacra. From each of these spiracles, a 
canal passes under the test in a dmection away from the mouth. This canal soon 
branches, and a branch goes to the side of each ambulacrum. Each branch of the 
canal swells into a pouch with thin walls that are strengthened by a slight deposit 
of lime; and these walls are thrown into folds so that their surface is increased. 
There is thus a folded pouch running along the inside of the test under each side 
of an ambulacrum; and from this pouch short tubes are given off which open to 
the exterior through pores at the sides of the ambulacrum, which pores alternate 
SWELLINGS IN THE PINNULES OF CRINOIDS PRODUCED BY A 
parasite (twice nat. size). 
