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. 



SWELLINGS IN THE PINNULES OF CRINOIDS PRODUCED BY A 



parasite (twice nat. size). 



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 direction 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 



