14 Prof. J. Miiller on the Structure of the Ecliinodernis. 



calcareous plate dividing and sending a ray to each digitation. 

 3. Tactile feet, whose expanded end is peuicillate, being closely 

 covered with stalked knobs ; the stalks contain a simple calca- 

 reous rod. 4. Gill-like feet, ambulacral gills; three-cornered 

 laminae pointed at their ends and having their edges pectinated 

 by processes or insections. Two or even three kinds of feet are 

 disposed on the same radius in those portions of the ambulacra 

 which are bounded by the semita. 



The import of the semitce of Philippi, the fascioles of Agassiz 

 — structures peculiar to the Spatangida — has been hitherto un- 

 known. They are distinguished from other parts of the shell by 

 bearing fine ciliated bristles instead of spines, the outer surface 

 of which up to the soft, knobbed, outer extremity of the bristle 

 exhibits a lively ciliary motion. The semita are therefore ciliated 

 fringes upon determinate arese of the shell. (Archiv 1853, p. 1.) 



In the genus Spatangus we meet with three kinds of feet ; 

 tactile, locomotive, and branchial feet, with regard to which I 

 may refer to JDuvernoy's beautiful figures. In all the ambulacra, 

 those feet which are nearest the mouth, the oral tentacles, are 

 covered at their extremities with clavate cirri ; the other ventral 

 feet are locomotive without any cap of cirii. In the subanal area, 

 on the other hand, that is, within the area circumscribed by 

 the subanal semita, there are on each side three additional cir- 

 rated feet which Duvernoy has overlooked. Delle Chiaje, on 

 the contrary, has improperly given cirri to all the ventral feet in 

 his figure of Brissus Scillce. The subanal cirrated feet belong to 

 the two posterior ambulacra and in fact, to their inner halves, so 

 that the subanal semita passes between the internal and the 

 external halves of the ambulacrum. The dorsal feet of the four 

 petaloid ambulacra are gill-like. The anterior radius has no 

 ambulacral gills at all ; but the locomotive feet, preserving the 

 same form, extend to its upper extremity; this radius is there- 

 fore rightly distinguished by Duvernoy as radius locomotorius. 

 The genus Spatangus possesses no semita upon the upper sur- 

 face of its shell. In those genera which possess one, either cir- 

 cumscribing the dorsal part of all the ambulacra, as in Brissopis, 

 Schizaster, &c., or that of the anterior ambulacrum alone, with 

 the apex [Amphidetus, &c.), I find a peculiar kind of feet in the 

 upper part of the anterior radius also, i. e. locomotive feet, with 

 discoid or stellate digitated suctorial plates. The figure of Brissus 

 Scilla by Delle Chiaje, again, shows nothing of this difference ; he 

 even represents the cirrated oral feet extending along the anterior 

 radius as far as the semita. It is improbable that Brissus should 

 differ from Brissopis, and the figure would appear to be in fault. 



In Brissopis and Schizaster the peripetalous semita divides 

 the anterior radius transversely into two portions ; the region 



