1861.] ANATOMY OF THE ECHINODERMS. 55 



all forty separate parts. The alveoli, when separated, are seen to 

 be triangular in shape : they have a broad external rounded surface, 

 presenting a deep hoUov? excavation ; at the bottom of this is a groove 

 in which is inserted a muscle. The surfaces opposed to the next pair 

 of alveoli are finely striated ; and these striae may be seen to be con- 

 tinued as free points, forming a finely toothed margin on the oesopha- 

 geal border ; to each of these surfaces a muscle is attached, passing 

 from one to that adjoining in the next pair. The remaining surface 

 presents the half of the groove for the tooth, which is completed by 

 the other alveolar piece, and also the symphysis that unites the two 

 portions. The epiphyses arch over the upper or basal part of the 

 alveoli on their outer borders, and serve as attachments for muscles. 

 The radii are long, slender, arcuated portions, situated on the upper 

 part of the oral skeleton between the pairs of epiphyses of the alveoli 

 and above the falces : the oesophageal end of each is articulated with 

 the centre of the inner end of the falces ; this portion is rounded ; 

 but just above the point at which they are divided transversely they 

 are laterally compressed, and here a muscle is inserted ; they are 

 then rounded again, and end by a flattened and bifurcated free ex- 

 tremity, forming points for the attachment of the tendons of slender 

 muscles. The falces are somewhat square portions, which fit accu- 

 rately between the pairs of alveoli, at their base ; the oesophageal 

 end has a deep notch, to the centre of which the radius is articulated, 

 and on either side of this the oesophageal ligaments are attached. 

 The teeth, five in number, are in shape somewhat like the incisor 

 tooth of a Rodent ; they have a hard, triangular, pointed prehensile 

 end, and towards the root they become gradually soft and friable, 

 and, when dry, split up easily into fine silky fibres ; they are covered 

 at their roots by a loose bag or pouch of the membrane of the peri- 

 some, which also connects the intervals between the muscles and the 

 various parts of the splanchnic skeleton. The alveoli and teeth are, 

 when in natural position, inter-ambulacral, the radii and falces being 

 ambulacral. 



With regard to the numerous muscles supplied to the apparatus, 

 amounting to forty in number, there are first to be mentioned ^«e 

 ^aiVsof what may be cTuWedi protractores (PI. X. A. fig. 1 a), arising 

 from the interambulacral region of the oral edge of the corona, and 

 inserted into the upper and lateral borders of the epiphyses of the 

 alveoli, and into a groove on the external surface of the alveoli them- 

 selves : their office is thus plainly seen, when acting together, to be 

 to protrude the points of the teeth — or as protractors ; and when 

 acting singly, to draw the teeth to one side or another. There are 

 Jive pairs of muscles arising from the inner surface of the auricular 

 arches, and inserted into the oral ends or apices of the alveoli, on the 

 inner side of these (PI. X. A. fig. 1 b) ; they are obviously retractors 

 (retractores), and antagonistic in their action to the preceding set. 

 Another set of five pairs of slender muscles arises immediately in 

 front of the protractor group, by a thin, narrow common origin, and 

 terminates by two slender tendons which ]iass obliquely to be inserted 

 into the bifurcated free ends of the radii. These are the radial muscles 

 or radiates (PI. X. A. fig. 1 e) : their action is to bring the points of 



