Prof. J. Miiller on the Structure of the Echinoderms. 17 



(together with pedicellarije and spines). When the petaloid am- 

 bulacra terminate^ their internal pore-area extends peripherally as 

 far as the edge. In Clypeaster it again occupies a portion of the 

 inter-ambulacral plates, while in Arachnoides it is excluded from 

 them. The pores of the locomotive feet are most readily observed 

 dpon the inner surface of the shell when this is naked ; in the 

 large Clypeasteridce they may also be easily detected on the 

 exterior with high magnifying powers, and still more readily in 

 the pore-fascijB of the other section, in which they are round and 

 simple. In the Clypeasters, where the internal apertures of the 

 locomotive pores are also simple, their external openings are 

 elongated, usually figure-of-8-shaped, and not uncommonly 

 divided into two distinct pores. In Clypeaster rosaceus these 

 pores are -yy to gy long, and -g-y broad. 



Upon the internal area of the petaloid ambulacra, the locomo- 

 tive pores are arranged in such a manner, that their longitudinal 

 diameter is directed radially, a disposition which they retain 

 until close to the peripheral edge ; the lateral ones, however, in 

 the peripheral portion of the shell have a somev/hat oblique di- 

 rection, and become more and more so the more external their 

 position ; the direction of the axes of the pores in fact is from 

 the periphery upwards and outwards. Upon the ventral surface of 

 Clypeaster, the similarly constructed elongated pores are all dis- 

 posed obliquely, viz. instead of taking a direction from the oral 

 centre towards the periphery, they diverge from the middl line 

 of the ambulacrum -, that is, if we produce the longitudinal axis of 

 each single locomotive pore, it will cut obliquely the middle line 

 of the ambulacrum, and form an acute angle open towards the 

 periphery. It is only towards the edge that the middle portion 

 of the series becomes straighter. The lines of direction of the 

 axes of the locomotive pores on the back are the continuation of 

 the same lines of direction on the ventral surface. What are 

 here denominated lines of direction of the axes of the pores must 

 not be confounded with the lines upon which the pores are ar- 

 ranged, for these in the Clypeasters are never disposed in series, 

 but are scattered without order. On the other hand, in Arach- 

 noides, the pores, as internally they are disposed in transverse 

 series, so, externally, they are arranged in oblique parallel rows, 

 whose parallelism is continued from the ventral to the dorsal 

 side, and through the whole extent of the petaloid ambulacra. 



In EcJiinarachnius parma, according to Agassiz, who has ex- 

 amined the living animal, the locomotive feet are replaced by 

 tentacles, which, without being connected with the ambulacral 

 vessels, open through their ampullae into the abdominal cavity. 

 The pores for these tubes are found in the petaloid ambulacra in 

 regular rows internal to the great pores ; through these tubes he 



Ann. ^ Mag. N. Hist. Ser. 3. Vol.\m. 2 



