]() ECHTNODERMS OF THE BRITISH ISLES 



2. A strongly (IcvcIoixmI, regularly arranged calcareous skeleton 



present ; body star-siiapcd, globular or egg-shaped . . 3 



No strong, regularly arranged skeleton, at most imbricating 



scales, and in this case with a naked, flat ventral disk. Body 



sausage -shaped Sea-cucumbers or Holothurians (Holothurioidea) 



3. Body star-shaped, usually with distinct arms ... 4 

 Body globular or egg-shaped or flat ; no arms 



Sea-urchins or Echinoids {Echinoidea) 



4. Arms long and slender, sometimes branching, sharply marked 



off from the disk. Tube -feet not in an open groove on the 

 ventral side of the arms 



Brittle-stars or Ophiuroids (Ophiuroidea) 

 Arms not sharply marked off from the disk, never branching. 

 Tube -feet in an open groove on the ventral side of the arms 



Sea-stars or Asteroids {Asteroidea) 



I. Class. SEA-LILIES or CRINOIDS 



(Crinoidea) 



Cup-shaped echinoderms with feather-like arms ; attached, 

 constantly or only in the young stage, by means of an 

 articulated stalk, arising from the dorsal side of the body. A 

 long, coiled intestinal canal ; stomach not sac-shaped. Mouth 

 and anal openings on the ventral side, which is turned upwards. 

 Ambulacral grooves open.^ 



The stalk of the sea-lilies known to occur in the British seas 

 is thin and slender, with root-like, branching appendages. In 

 other forms (family Isocrinidae) it is very stout, pentagonal, with 

 articulate, unbranched appendages, cirri, arranged in whorls. In 

 feather-stars (Comatulids) cirri occur only^ on the uppermost 

 stalk-segment, which is strongly developed, and is termed the 

 centro-dorsal. The cirri are attached, each in a small groove, the 

 cirrus-socket, on the centro-dorsal. The shape and arrangement 

 of these sockets is of systematic importance, as the centro- 

 dorsal usually, through its great variation in regard to shape 

 and size, affords characters of much systematic value within the 

 Comatulids. This also holds good for the cirri, which show 



^ In literature on Echinoderms the designations " actinal " and 

 " abactinal " are very commonly used for " ventral " and " dorsal ", as 

 also the term " actinostome " for the mouth. The author does not see 

 any necessity at all for using these artificial terms, and therefore uses the 

 simple, unmistakable designations " ventral " and " dorsal ", or " oral " 

 and " ahoral ". 



