8 ECHINODERMS OF THE BRITISH ISLES 



Anterior part of filiated band separated from the rest ; no 

 calcareous bodies . . Asteroid larvae, Bipinnaria 



3. Skeleton consisting of two symmetrical halves ; posterolateral 

 arms directed forwards, strongly developed 



Ophiuroid larvie, Ophioplutcus 

 Skeleton consisting in I. stage of two, in II. stage of four paired 

 and one or two unpaired parts ; posterolateral arms wanting 

 or directed backwards or to the side 



Echinoid larvae, Echinopluteus 



In addition to sexual reproduction several echinoderms have 

 another way of propagating, viz. by self -division, each half 

 regenerating the lacking part and thus developing into a complete 

 individual. Among British species this appears to be normally 

 the case in Cucumaria lactea, as it is in the Mediterranean species 

 Ophiactis virens and Coscinasterias tenuispinus and two species from 

 the Scandinavian seas, Ophiactis nidarosiensis and P solus valvatus. 

 By such division the two parts are always completely severed from 

 each other ; colony formation is unknown among echinoderms. 



This peculiar mode of propagation has some relation to the 

 fact that regenerating power is, on the Avhole, very strong 

 in echinoderms. Sea-stars and brittle-stars with regenerating 

 arms are very commonly met with. Some brittle-stars (esj^ecially 

 Amphiura filiformis among British sj)ecies) may throw off the disk 

 (Avith the stomach and the genital organs) and regenerate it all. 

 The same holds good of the sea-lilies. Most sea-cucumbers may, 

 by strongly contracting the body wall, eject the w^hole of their in- 

 ternal organs through the anal opening, and regenerate all again. 



The echinoderms occur in all depths of the sea, from 

 the shore to several thousand metres. They are very 

 sensitive to brackish water, which accounts for the fact that 

 the whole of the eastern Baltic is quite devoid of echinoderms. 

 No echinoderm is found in fresh water. Some species occur 

 in such great numbers as to be among the main characteristics 

 of various animal communities of the sea {e.g. Echinocardium 

 cordatum, Amphiura filiformis, Ophiura affinis). Upon the whole, 

 the echinoderms play an important role in the economy of the 

 sea, partly because they devour great quantities of other animals, 

 mussels, snails, worms, etc., partly through serving as food to 

 various species of fish, e.g. cod, haddock, plaice, dab. Also some 

 sea-birds, mainly in the Arctic regions, eat echinoderms, 

 especially the sea-urchin Strongylocentrotu^ drobachiensis. In 

 more southern regions some species of sea-urchins are highly 

 esteemed as food for man (only the genital organs are eaten). 



