PEIMARY LARVAL PLATES OP BRAOHIATE EOHINODERMS. 27 



Encrinus, Cyathocrinus, &c.), but which are not repre- ^ 



sented in the Com a tula larva. The under-basals are marked ---^ 

 1, %, in the diagram. At the extremity of the rays are found 

 the odd terminal plates t, t. These plates, in my opinion, 

 have no representatives in the Crinoid. 



Ludwig in his admirable memoir on the development of 

 the Ophiurid skeleton,i states that he is unable to say positively 

 whether the terminal or the first radial plate is the first formed ; 

 and it is unquestionable that there is very little difi'erence in the 

 time of the appearance of the two sets of plates in the species 

 he studied, Amphiura sq nam at a, — an example, it may be 

 remarked, of a viviparous Ophiurid in which the development 

 is abbreviated. As far as judgment can be drawn from the 

 the figures given by Agassiz,^ ISIetschnikoff,^ and Miiller^ of 

 larval Ophiurans which pass through a pluteus stage it seems 

 more probable that traces of the terminal plates appear before 

 the first radials are formed. Although in the case of 

 Amphiura studied by MetschnikoflF (op. cit., Taf. iv, fig. 17), 

 and perhaps in some of Miiller^s figures also, the reverse would 

 seem to occur. The relation of the first radials to the water- 

 vascular system in Metschnikoff's figure is very noteworthy. 

 The question, however, whether the terminals or the plates 

 which we have called first radials in the Ophiurid are the first 

 to appear, has, in my opinion, upon phylogeuetic and embryo- 

 logical grounds, nothing whatever to do with the determination 

 of the bomologues of the Crinoid first radial, although some 

 writers would seem to infer such a view by saying that in [ 

 Asterids and Ophiurids the first radial, (sic) is carried out j 

 during growth to the extremity of the rayS A ti^Jj<- 



The primitive structure and mode of formation of the ' 

 terminal plate is different from that of the first radial. 



1 'Zeitschr. f. wissenscli. Zool.,' Bd. xxxvi; * Morphol. Studien,' ii, p. lOi. 



' " On the Embryology of the Echinoderms," fig. 32, ' Mem. Amer. Acad.,' 

 1864. 



3 " Ueb. die Entwickeluug der Echinodermen u. Nemertiiien," Taf. vi, vii, 

 ' Mem. Akad. St. Petersb.,' 1869. 



" " Ueb. Opliiurenlarvea des Adriat. Meeres," Taf iii, iv, vii, ' Berlin Akad.,' 

 1851. 



