152 



ECHINODERMS OF THE BRITISH ISLES 



dissolve the vertebraB so that the arms break ; where present in 

 great quantity it no doubt causes the death of the animal. 



Fossil Ophiuroids are on the whole rare and, excepting the 

 Palaeozoic forms, not of great importance, either from a morpho- 

 logical or from a geological point of view. 



In the viviparous Ophiuroids (see above, p. 149) development 

 takes place within the bursae, and the young ones remain there 

 until they have reached such a considerable size that it is almost 



Fig. 87. — Skeleton of various Ophiuroid larvae (Ophiopluteus). 

 (From Danmark''s Fauna.) 



1. Ophiocomina nigra (xlia). 2. Ophiopluteus ramosus{x 100). 3. Ophiothrixfragilis 

 ( X 100), 4. Ophiopluteus dubius, posterior part of skeleton only ( x 100). al, Antero- 

 lateral rod; e, End rod ; k, Body rod; pd, Postero-dorsal rod ; pi, Postero-lateral rod; po, 

 Postoralrod; pr, Median rod ; r, Recurrent rod ; t, Transverse rod. In 2 the recurrent rod 

 of only the one (ventral) side is drawn ; a corresponding one is found on the other (dorsal) side, 

 the body skeleton thus forming two large, rectangular meshes in each side of the body. 



inconceivable how they can get out through the narrow genital 

 slits. Most Oj^hiuroids, however, have free-swimming, pelagic 

 larvae, Ophiopluteus, of a very characteristic shape, with long 

 processes or " arms ", among which the posterior lateral arms are 

 the longest, and directed more or less obliquely forwards. They 

 have a well-developed skeleton, consisting of two symmetric 

 halves, joining in the posterior part of the animal (Fig. 3, 3 and 

 Fig. 87). From the body-rod, which is either simple or compound, 

 forming two meshes in each side of the body, proceed rods that 



