THE CRINOIDEA. 



497 



dium is a Pluteus, and has a skeleton formed of calcareous 

 rods, which support the processes into which the body, in the 

 region of the ciliated bands and elsewhere, is prolonged. 



The origin of the ambulacral system, before it has the 

 form of a caecum with a dorsal pore, has not been made out. 

 The blind end of this caecum lies on the left side of the ali- 

 mentary canal, and is connected with a discoidal body, which 

 is situated on the left side of the stomach ; a similar body ap- 

 pears on the right side. Doubtless these discoidal bodies an- 

 swer to the peritoneal diverticula of the alimentary canal of 

 the Echinopaedium in other Echinoderms. 



The blind end of the tube enlarges, and gives rise to a 

 rosette, whence the ambulacral vessels proceed ; and a de- 

 pression of the integument of the larva, forming the so-called 

 umbo, extends inward to this. At the bottom of the umbo, 

 a new mouth opens through the centre of the rosette into the 

 gastric cavity of the larva, the primitive oesophagus being 

 abolished. The larval skeleton undergoes resorption, but the 

 rest of the Echinopaedium passes into the Echinoderm. 1 



Loven has recently drawn attention to the fact that, in 

 young Echinids, 2 the plates of the apical region are not only 

 more conspicuous in relation to the corona, but differ some- 

 what in their arrangement from those of the adult. Thus 

 the anus is at first wanting, and the anal plate, which occu- 

 pies the centre of the apical area, is relatively large ; it is 

 united by its edges with the five plates, which, imperforate in 

 the young, will become the genital plates in the adult. The 

 five ocular plates are also imperforate, and are disposed in a 

 circle outside that formed by the genital plates, their inter- 

 spaces being occupied by interambulacral plates. The apical 

 region of an Echinid has thus, as Loven points out, a most 

 striking resemblance to the calyx of a Crinoid ; the anal 

 plate representing the hasalia, the genital plates the para- 

 basalia, and the ocular plates the first radialia. 



The Ckinoidea. — This remarkable group, which abounded 

 in former periods of the world's history, is represented at the 



J See, in addition to the memoirs of Miiller and Metschnikoff already cited 

 A. Agassiz, " On the Embryology of Echinoderms." (" Mem. American Acad- 

 emy of Sciences," 1864.) 



v i? T he admirable monograph of A. Agassiz, " Revision of the Echini," pub- 

 lished in the " Illustrated Catalogue of the Museum of Comparative Zoology at 

 Harvard College," is also full of information respecting the young states of'the 

 Echinids. ° 



