XXI 



LARVAL FORMS 



607 



Dipleurula. He named this one Pluteus (easel), from a fancied 

 resemblance, when turned upside down, to a painter's easel. The 

 same name was bestowed on the next type, to which it presents 

 a superficial resemblance, and hence the distinguishing prefix 

 " Ophio- " was added to the original name by Mortensen. 



(3) The Uchinophiteus, the larva of the Echinoidea. This 

 type is strikingly like the preceding one in possessing a -very 

 small prae-oral lobe and in having the processes of the ciliated 

 ring supported by calcareous rods, but a close inspection of these 

 shows that they do not exactly correspond to those of the 

 Ophiopluteus. Thus we have prae-oral, postero-dorsal, and post- 



antero -lateral 

 arm 



post-oral arm 



Fig. 285. Dorsal view of 

 larva of Echinus escu- 

 lentus. x 45. a.cil.ep, 

 Anterior ciliated epau- 

 lette ; ap, apical plate 

 or larval brain ; ech, 

 rudiment of Sea- 

 urchin ; Lax, left an- 

 terior coelom ; l.oes, 

 larval oesophagus ; 

 l.p.c, r.px, as in Fig. 

 284 ; pxil.ep, posterior 

 ciliated epaulette ; 

 r.a.c, right anterior 

 coelom. 



p-ciKeg 



oral processes (Fig. 285), but usually no postero-lateral process, 

 and when it does occur it remains short. On the other hand, 

 an antero-lateral process unrepresented in the Ophiopluteus is 

 constantly present, and in its later stage the Echinopluteus 

 develops, out of parts of the ciliated ring, horizontally-placed 

 crescentic ridges of cilia, which are termed ciliated epaulettes 

 (Fig. 285, a.cil.ep). There may even be, as in the larva of 

 Echinus esculentus, a second posterior set of these (Fig. 285, 

 p.cil.ep). In the older larva at the apex of the prae-oral lobe 

 there is an ectodermic thickening, at the base of which are 

 developed nerve-cells and nerve fibres constituting a larval 

 brain (Fig. 285, ap). 



(4) The Auricularia, the larva of the Holothuroidea. This 

 type strikingly resembles the Bipinnaria in its external features. 

 The prae-oral lobe is well developed, and has on its under surface 

 a backwardly projecting loop of the ciliated band, which is not, 



