MARINE BIOLOGICAL STATION AT PORT ERIN. 81 



Ophiurids, the sand-stars and brittle-stars, such as 

 Ophiothrix fragilis (fig. YIIL, 3), have no 

 suckers on their tube-feet, and can be seen to have 

 an interesting new method of locomotion peculiar 

 to themselves ; they jerk their bodies along by 

 alternately curving and straightening their 

 muscular many- jointed but fragile rays. 



Echinids, Sea-urchins. In addition to the " regular " 

 urchins, such as the large pink Echinus esculentus 

 (fig. VIII., 5), found on the rocks and breakwater 

 at low tide, and the smaller dull green E. 

 milliaris, found in the rock pools, there are at 

 Port Erin several kinds of the delicate " heart " 

 or " oblique " urchins. The commonest of these 

 is Echinocardium cor datum, which burrows a few 

 inches below the surface of the sand at low tide 

 in the centre of the beach. 



Holothurids, Sea-cucumbers. The elongated, worm- 

 like body has a mouth surrounded by a crown of 

 tentacles at one end, while rows of sucking tube- 

 feet run down the 5 angles of the body. The 

 skin is strengthened by calcareous plates, often of 

 delicate and beautiful shapes under the micro- 

 scope. Cucumaria upland (fig. VIII. 4) is 

 dredged from deep water ; another kind, Synapta 

 digitata, which has anchor-shaped spicules in the 

 skin, and no tube-feet, is found burrowing in 

 sandy gravel at low tide, in front of the 

 old Biological Station. 



The eggs of all these common Echinoderms develop 



first into minute larvae which are quite unlike the parents 



in appearance and structure, and are found floating on the 



surface of the sea. A larval Ophiurid, called " Pluteus," 



G 



