48 ECHINODERMS OF THE BRITISH ISLES 



the imul of the sea bottom, assimilating the organic substances 

 and small organisms {e.g. Foraminifera) found therein. In some 

 forms {Ctenodiscus, Astropecten, Porania, and others) ciliary 

 movements on the oral side and in the ambulacral furrows play 

 some part, leading a stream of microscopical particles to the 

 mouth. Otherwise the main purpose of the ciliary currents is 

 respiratory and cleansing. 



The sea-stars are of no use to man ; they are not used as food, 

 such as is the case with sea-urchins and sea-cucumbers. Where 

 they occur in great numbers, e.g. Asterias rubens in some Danish 

 fjords, they might perhaps advantageously be used as manure. 



It appears that sea-stars take, as a rule, about four years in 

 reaching full size. For the present, however, little is known with 

 certainty about this matter, as also nothing is known about how 

 long a sea-star may live. With regard to the common sea-star, 

 Asterias rubens, it is an established fact that the more abundant 

 its food is, the faster it grows ; it has also been shown that its 

 rate of growth is faster in summer than in winter. The closely 

 related American species, Asterias vulgaris, has been shown to 

 reach a size of somewhat beyond 5 cm. length of arm in the 

 course of four months, when a rich supply of food is available 

 to it ; probably the same will hold good for Asterias rubens. At 

 least the larger specimens of this species are sexually mature when 

 one year old. 



The regenerating power is very great. Some species {e.g. 

 Marthasterias glacialis, PediceUaster typicus) amputate their 

 arms close to the disk when they are hurt or are held under un- 

 favourable conditions. This to a special degree holds good of 

 Brisinga, which will very often amputate all its arms on being 

 brought to the surface, so that one finds only the small disk and 

 a number of isolated arms. Several forms, especially of the 

 many-rayed {e.g. Coscinasterias tenuispina), divide through 

 autotomy in two halves, each half then regenerating the lacking 

 parts. In some tropical forms {Linckia) an amputated arm may 

 regenerate the whole sea-star. 



Parasites are not found in great numbers in Asteroids. A 

 species of Myzostoma lives as entoparasite in the pyloric 

 caeca of Asterias Richardi. A polychsetous annehd of the 

 Lepidonotids, Acholoe astericola, lives in the ambulacral grooves 

 of Astropecten. Various species of parasitic snails {Thyca, 

 Stylifer) are found on some tropical sea-stars. Some haK-parasitic 

 Copepods occur on Asteroids {Astericola Clausi on Marthasterias _ 

 glacialis, Astrochaeres Liljeborgi on Henricia sanguinolenta) . A 



