16 Prof. J. ^liiller on the Structwe of the Echinoderms. 



more so than in the regular Sea-urchins^ where their number 

 amounts usually to about 2000, or in the Spatangiday which 

 possess only a few hundred ; in the Clypeasters the number of 

 locomotive feet may, without fear of exaggeration, be estimated 

 at many myriads. They are provided with a sucker at thiir 

 extremities, which is either supported by a notched calcareous 

 ring, as in the Clypeasters and their allies, or contains at least a 

 couple of calcareous bodies, as in Mellita, where at the base of 

 the sucker we find in general two scale-beam-shaped (wage- 

 balkcnformig) calcareous rods, with two long and one short 

 process, lying opposite to one another. Mellita hexaporu, 

 Agassiz. The locomotive feet extend for a greater or a less 

 distance from the ventral to the dorsal surface. Their distribu- 

 tion upon the abdominal surface varies greatly, whence the Cly- 

 peasters may be divided into two sections. In the first, the feet 

 are not distributed over the whole of the ambulacral plates, but 

 occupy separate branched passages in which the pores are aggre- 

 gated, the pore-fascice. These fasciae are at first simple, then 

 divide dichotomously or trichotomously {Echinarachnius), and 

 their principal branches ramify again for the most part into 

 lateral branches. Agassiz has already observed that these branches 

 also pass on to the inter-ambulacral plates. To this section 

 belong the genera Rotula, Mellita, Encope, Lobophoraj Scutella, 

 Echinarachnius. In the other section of the Clt/peasteridce, 

 including the genera Clypeaster, Laganum, Arachnoides, Mou- 

 linia, Scutellina, EchinocyamuSj Fibularia, the pore-fascise are 

 entirely absent. A few have, indeed, in the middle line of the 

 ambulacra a simple groove, as Arachnoides, but it is known to 

 be without pores. The distribution of the pores and feet in 

 these genera has hitherto been unknown, but may be determined 

 with certainty in the larger kinds. The pores and feet are scat- 

 tered over the entire surface of the ambulacral plates ; and in CVy- 

 peaste?^f but not in Arachnoides, they pass on to a considerable 

 portion of the inter-ambulacral plates. These, therefore, are 

 Clypeasteridce with pore-arece, in opposition to those with joore- 

 fascicR. The division into Clypeasteridce with simple, and those 

 with branched grooves, does not seize the real distinction. A 

 few genera with pore-arece have the middle of the pore-area not 

 even depressed, as Echinocyamus and Fibularia. In Laganum, 

 indeed, the depression is half lost. 



Upon the dorsal side of the Clypeasteridce we must distinguish, 

 in the petaloid ambulacra, the external area between every two 

 series of double pores and the internal area. The former is 

 always provided with ambulacral gills appertaining to the large 

 double pores ; the latter in many genera, Clypeastei', Arachnoides, 

 Echinarachnius, is covered with the very small locomotive feet 



