8 ON THE EMBRYOLOGY OF ECHINODERMS. 



The remaining obsei'vations of this paper were all made from specimens caught 

 swimming on the surface of the water ; this applies to the fully-formed Sea-urchins 

 as well as the larvae, only the most advanced specimens (fig. 28) were found thrown 

 np on the beach after a storm, attached to Laminaria. The specimens obtained in 

 this way, the various stages of which were traced until there could be no doubt to 

 what species they belong, connect so nearly with specimens obtained from artificial 

 fecundation as to leave but few gaps to fill, in the larval condition, to give us all 

 their transformations. The Sea-urchins raised from larvae caught swimming freely 

 about were kept in confinement until they had attained the size of some of the more 

 advanced nomadic Sea-urchins, figs. 26, 27. This can leave no doubt to which of our 

 two species of Echinoids these larvae should be referred. 



The next oldest larva, which is fig. 19, shows that since the last stage represented 

 the principal changes have taken place in the oral part of the larva, fig. 19; the arms 

 e\ e" especially have increased greatly in length, the outline of the anal extremity is 

 somewhat rounded, the rod which runs along its edges is made up of short, stout pieces 

 with strong pointed projections, and the rods of the arms are composed of three rods 

 connected together by transverse spokes ; it requires close examination to distinguish 

 this. On the aboral side two very prominent spurs project over the stomach, somewhat 

 below the point of junction of the rods of the arms e', e", e". Additional tentacular 

 loops have been formed ; we can distinctly trace three on the surface of the left water- 

 tube w' ; the outline of a part of the right water-tube [w) shows great increase in the 

 volume of the tube. In an adult Sea-urchin Pluteus, fig. 20, the Sea-urchin has 

 encroached so much on the anal extremity as to conceal the shape of the digestive 

 cavity. The spines are so large, that we are unable to trace the position of the tentac- 

 ular system ; the anal opening is very conspicuous. The vibratile epaulettes, v", are 

 remarkably powerful. The arms have attained nearly the same length. The vibratile 

 chord has been twisted in such a manner as to assume the appearance of binding an 

 anal and an oral plastron, of Avhich e', e", e'", and e" are respectively the arms ; the 

 mode of formation of the chord and of the arms shows that all these arms in reality 

 belong to one plastron (see figs. 12, 13), notwithstanding the great resemblance to the 

 two distinct plastrons of a Brachiolaria. Two very prominent black spots are seen in 

 the arms e', e", similar to those observed by Miiller in his Pluteus qiiadrimaculatus ; a 

 few small spots are scattered over the other arms. The Pluteus figured here in its 

 natural attitude does not undergo any further changes of form ; it now enters a stage 

 when the Sea-urchin goes through its greatest transformations ; these unfortunately 

 cannot be followed, owing to the opacity of the larva. 



