68 ECHINODERMS OF THE BRITISH ISLES 



The larva (Fig. 28, 1, 2) is rather large, with the anterior jmrt 

 of the body greatly prolonged, all the arms being placed close 

 together in the posterior part and around the mouth. It appears 

 to be a special character of these larva? that the left postero- 

 lateral arm is distinctly shorter than the right. Furthermore, the 

 Luidia larvae differ from other Asteroid larva? in swimming not 

 only by means of the ciliated bands, but also by actively 

 moving the two median processes that terminate the elongated 

 anterior jmrt of the body. When the young sea-star is ready for 

 living by itself, the whole larval body is thrown off ; the latter 

 may still go on swimming about for some time. In other Asteroid 

 larvae the larval body is not thrown off but completely absorbed 

 by the young sea-star. The .young sea-star hanging on the larva 

 is red, the larva otherwise colourless. Apparently there is no 

 other character than the size by which to distinguish the two 

 species of Luidia larvae, the fully-formed larva of L. Sarsi being 

 ca. 25-35 mm. long, the larva of L. ciliaris only ca. 7 mm. ; 

 however, this is not very certain, the larva of the latter species 

 having not yet been very carefully studied. As soon as 

 the h^^drocoel has formed the first rudiments of the radial 

 water - vessels, the tw^o larvae are easily distinguished, the 

 larva of L. Sarsi having only 5, that of L. ciliaris 7, radial 

 pouches. 



The Luidia Sarsi larva was the first echinoderm larva to be 

 discovered, viz. by Michael Sars, who in 1835 described it under 

 the name of Bipinnaria asterigera, without, however, suspecting 

 its being an Asteroid larva. Only many years later Ludwig 

 recognised it as the larva of Luidia Sarsi. 



The number of eggs that an adult Luidia ciliaris produces 

 yearly is immensely great, at least ca. 200 millions. As this 

 species is, however, not represented by any great number of 

 individuals, even where it is most common {e.g. the Channel), 

 this necessarily means that an immense number of the eggs and 

 larvae must perish. 



The rather numerous species known of the genus Luidia are 

 subdivided in several subgenera. The two European species 

 belong to the subgenus Hemicnemis Miiller and Troschel. 



Key to the British species of Luidia. 



5 arms ; lower marginals with 3, rarely 4, spines . \. L. Sarsi 



7 „ ,, ,, ,, 4 or 5 spines . . 2. L. ciliaris 



