254 ECHINODERMS OF THE BRITISH ISLES 



joint. Ventral plates present only on the two proximal arm- 

 joints ; they are small, pentagonal, separate. Tentacle pores 

 only on the two proximal joints, covered by a fairly large, leaf- 

 shaped tentacle scale. Usually 6-8 short, rudimentary arm 

 spines. Mouth shields small, triangular ; outside the mouth 

 shield a large pentagonal plate, which occupies nearly the whole 

 interradial space, between the genital scales. Genital slits very 

 short, reaching only to the proximal end of the first side arm 

 plate, without papillae along the edges. The whole underside 

 of disk and arms very flat ; arms rather slender, but rigid. 

 Colour in life yellowish-brown. Grows to a size of ca. 35 mm. 

 diameter of disk. Arms ca. six times as long as diameter of disk. 



Biology and development unknown ; but it seems certain 

 that it takes 3-4 years to reach full size. 



In British seas this species was first found by the " Porcu- 

 pine ", but there is uncertainty about the exact locality where 

 it was taken. Later on it w-as taken by the '' Helga " off S.W. 

 Ireland, 1620-1768 m., sometimes in great numbers, and by the 

 " Michael Sars " at 57° 41' N., 11° 48' W., 1853 m. It is elsewhere 

 widely distributed both in the Atlantic and the Indo- Pacific ; a 

 truly cosmopolitan deep-sea form. Bathymetrical distribution 

 130-3435 m. 



IV. Class. SEA-URCHINS or ECHINOIDS 



(Echinoidea) 



Free-living echinoderms, with globular, egg-shaped, or 

 flattened body ; a compact, mostly hard test, formed by regu- 

 larly arranged plates, carrying spines articulated movably to 

 tubercles on the test. Tube-feet in 10 meridian series, not in 

 furrows ; a double pore in the test corresponding to each tube- 

 foot. Intestine long and coiled. Mouth turning downwards. 

 Anal opening always present. 



The bodyoi the sea-urchins is usually globular or egg-shaped ; in 

 the order of the Clypeastroids (mainly tropical forms) it is usually 

 flattened, often quite thin, disk-shaped. They may reach a con- 

 siderable size, up to about 8 inches in diameter. The skeleton 

 consists of very regularly arranged, usually pentagonal, plates, the 

 coronal plates, which are usually coalesced so as to form a hard 

 test. Only in some of the oldest, palaeozoic forms and in the 

 recent family of the Echinothuridae the test is thin and movable, 

 the plates being imbricated. The test (Fig. 139), or corona, is 



