BCnrNODERMATA. 



other case*, as A. ruliens, UK.TU art- two double 

 rows (Jig. 7, /),) in every ray, and each foot 

 has a round undivided vesicle (Jigs. 12 and 

 16, </). 



The canals or vessels which convey the 

 fluid to ami from the feet are all connected with 

 a circular vcsvd situated in the vicinity of the 

 mould. This vessel (jigs. 12 and 22, ',i,) lies 

 immediately within the calcareous ring already 

 described as connecting the rays at their com- 

 nii mement; from it a straight canal proceeds 

 along the floor of each ray in the median line, 

 and in its progress gives off lateral "brandies 

 which open into the vesicles of the feet. There 

 are moreover connected with the circular ves- 

 sel, first, a certain number of bodies (ten in 

 five-rayed species) which Tiedemann com- 

 pares to glands (figt. 12 and 22, m, TO); they 

 are very small, brown, sacculated organs, each 

 opening by a small orifice into the circular 

 vessel; Tiedemann supposes them to be the 

 source from which the fluid filling the feet is 

 derived. Secondly, pyriform sacs; inA.au- 

 rantiaca there are four groups of these (Jig- 

 22, k); and each group consists of three 

 or four sacs which open by a common 

 tubular pedicle into the circular vessel. In some 

 other species there are five simple sacs. They 

 are muscular, and Tiedemann conceives them 

 to be the chief agents by which the fluid is 

 forced into the vesicles of the feet, to which 

 they are placed in a sort of antagonism. It 

 would seem, however, that this purpose may 

 be accomplished by other means, for according 

 to Meckel's statement, and, we may add, our 

 own observation, they are not present in all 

 species. Lastly, the circular vessel receives 

 the singular organ named the stone canal or 

 sand canal by Tiedemann, (Jigs. 12 and 22, S,) 

 who describes it as a membranous canal con- 

 taining a friable mass of sandy or earthy 

 matter, which commences by a wide origin on 

 the inferior or internal surface of the calcareous 

 disk (jigs. 12 and 16, z,) already described 

 as situate on the upper part of the body, 

 descends in a duplicature of fibrous membrane, 

 and opens by a narrow orifice into the circular 

 vessel, the upper or wide end being closed by 

 the disk. Ehrenberg has correctly remarked 

 that this organ is not filled with an amorphous 

 mass of earthy or cretaceous matter ; he de- 

 scribes it as exhibiting a dense network of 

 calcareous fibres with hexagonal and penta- 

 gonal meshes, resembling in some respects the 

 cavernous structure of the penis. The result 

 of our own examination in more than one 

 species is different still. We have always 

 found the earthy matter forming a jointed cal- 

 careous tube. This tube, which is about the 

 thickness of a surgeon's probe, is composed 

 of rings of calcareous substance connected by 

 membrane, so that viewed externally it is not 

 unlike the windpipe of a small animal. On 

 cutting it across, however, it is found to be 

 more complex in structure than appears exter- 

 nally, for it contains within, two convoluted 

 lamina; of the same nature as its calcareous 

 |>arietes(./ig.l3). These laminae are rolled lon- 

 gitudinally; they rise conjointly or as one, from 



Portion of the sand 

 canal of Asteritu 

 mbfiu, mtujnijifil. 



Fig. 13. 



the internal surface of the 

 tube, pass inwardly a Ott- 

 tain way, then separating 

 are rolled in opposite di- 

 rections; something after 

 the same manner as the 

 inferior turbinated bone of 

 the ox. These internal 

 lamina' become more con- 

 voluted towards the upper 

 end, where at last they, as 

 well as the more external 

 part of the tube, join the 

 dorsal disk, appearing gra- 

 dually to become conti- 

 nuous with its substance. 

 The disk is perforated with numerous pores 

 which open into the tube. Tiedemann con- 

 ceives the function of the sand canal to be that 

 of secreting the earthy matter required for the 

 growth of the calcareous skeleton. Meckel 

 considered this view as very improbable, and 

 the description we have given does not tend to 

 corroborate it. We must confess ourselves 

 unable to offer more than mere conjecture as to 

 the use of this singular structure. If the fluid 

 contained in the feet and their vessels be sea- 

 water, (either pure or with an admixture of 

 organic particles,) which is probable from its 

 chemical composition, may it not be intro- 

 duced and perhaps again discharged through 

 the pores of the disk and the calcareous tube, 

 the porous disk serving as a sort of filter to 

 exclude impurities ? 



In the Echinus the feet are disposed in ver- 

 tical rows running from the anal orifice towards 

 the mouth; and the corresponding rows of 

 apertures (fig- 10, e, e,) thus diverging from a 

 point have been compared to garden-walks, 

 and named ambulacra. In most cases the feet 

 extend all the way to the inferior opening of 

 the shell, but in some genera they stop short 

 before reaching this point. There are ten rows 

 disposed in five pairs. The tubular part of each 

 foot communicates with the interior of the shell 

 by two branches which pass through two aper- 

 tures. These branches in some species (as 

 E. sexatalis) communicate directly with the 

 cauals which convey the fluid to the feet ; in 

 others (as E. esculentus) they open into a 

 plexus of vessels, by the intervention of which 

 they are connected with the canals. The plex- 

 uses of vessels alluded to are formed in leaf- 

 like membranes (Jig. 14,</, d, representing two 

 of them magnified,) which are of equal nuin- 



F.g.14. 



bcr with the feet, and of course disposed in 



i> 2 



