^42 Prof. J. Miiller on the Structure of the Echinoderms. 



of the Sea-urchins ; this is evident from the analysis of the Pen- 

 tremites given by Roemer. In Pentacrinus the antambulacral 

 and ambulacral zones of the calyx (with ambulacral grooves) are 

 equal, and both have become produced upon the moveable arms. 

 The development of the antambulacral side of the radii in the 

 Crinoids takes place either from the very base of the calyx, or 

 from its circumference, or in the neighbourhood of the mouth, 

 as in most Cystidece. In the latter case, the calyx presents no 

 radial arrangement of plates from the base to the immediate 

 neighbourhood of the mouth ; it begins only at the mouth in 

 the oral arms, whose ambulacral grooves however lead to the 

 mouth, and, like the articulated antambulacral surface of the 

 arms, present no traces of the general plan of the Echinoderms. 

 Hence it is intelligible why, so long as the Cystideans were held 

 to be armless, the radial arrangement of the Echinoderm was 

 unrecognised. 



The determination of the different structures which occur in 

 the^radii of the Crinoids is not always easy, bat I have come to 

 the following conclusions : — Radii are radial divisions of the 

 Crinoid for the reception of the ambulacra, and they are either 

 calycine radii or arms. Calycine ambulacra are grooves with 

 suckers upon the ventral surface of the calyx in the direction of 

 the arms when these exist. Ambulacra of the arms are the ven- 

 tral surfaces of the arms and pinnulse, provided with suckers. 

 The Blastoidea have calycine ambulacra without arms. Many 

 Crinoids, as Actinocrinus, Platycrinus, &c., have arms without 

 calycine ambulacra or clefts of the calyx ; the Pentacrinites and 

 their allies possess both arms and calycine ambulacra. The 

 arms are articulated in either one or two series, and are either 

 simple or dichotomously divided. The divided arms arise from 

 an undivided arm-basis, which is either articulated upon the 

 calyx or is enclosed by its plates. The arms therefore are pro- 

 bably not originally double. The pinnulce, on the other hand, are 

 always in double rows and are never divided or branched. 

 They are articulated processes either of the calycine ambulacra 

 (Blastoidea) or of the ambulacra of the arms. Where arms are 

 present the pinnulse are absent on the calyx, and first appear as 

 the arms become free from the calyx. They are articulated either 

 in single or in double series, and are provided upon their ventral 

 surface with suckers, like the arms and calycine ambulacra. 

 Every single joint of the arm, or every segment of the ambu- 

 lacrum {Blastoidea), has only one pinnula. The pinnulse always 

 alternate. Unquestionable examples of arms without articulated 

 pinnula are afforded by Cupressocrinites ; of pinnules without 

 arms by Pentremites, Pinnula composed of a single piece, form- 

 ing series upon a joint (of an arm) like the little plates on the 



