412 Mr. W. P. Sladen on the Structure of 



2. The limitation of the tentacular pore-system to the disk 

 likewise supports the above view as to the affinities of Astro- 

 phiura. An approach in the same direction, although in a 

 very different degree, is not wholly unknown in true Ophi- 

 urans, the tentacle-pore3 extending very little, if at all, beyond 

 the disk in Ophiomusium and Ophiolipus. 



3. The extremely rudimentary condition and aborted cha- 

 racter of that portion of the radial series which is prolonged 

 beyond the body-disk would seem to give indications of 

 disuse and cessation of function in this area of Ophiuroid 

 organization, followed by a localization of function according 

 to the plan of Asteroid organization. Perhaps, to a certain 

 extent, Ave find a step in the same direction in Ophiomusium 

 and a few of the Opliioglyphce. 



4. The extraordinary development of the tentacular or 

 ambulacral system compared with its usual standing in 

 Ophiuran anatomy, together with a most extreme modifica- 

 tion of the muscular system characteristic of that group, 

 indicate unequivocally a tendency towards the growth of 

 Asteroid characters ; whilst the septa or supplementary plates 

 which form the divisional partitions of the tentacular com- 

 partments in Astrophiura are not only unknown in Ophiu- 

 roidea, but, as far as I am aware, are confined to the Aste- 

 roidea. Regarding the aborted axial elements of the radii in 

 the present echinoderm as the natural homologues of the 

 ambulacral plates of Asteroidea, it remains only to determine 

 the equivalents of the septa or accessory plates ; and these I 

 propose to identify with the internal connective pieces which 

 occur in Astropectinidae and Linckiadse, and fill in the angle 

 formed by the ambulacral and ventro-lateral plates, and to 

 which M. Viguier (who has recently studied the calcareous 

 test of starfishes) has applied the name of " soutiens ambula- 

 craires." If the view just enunciated be correct, we are 

 presented by this plate in Astrophiura with the indications of 

 a stage in the genetic development of Asteroidea all traces of 

 which have gradually passed away in the course of the evolu- 

 tion of the more advanced forms of the group. 



It should be mentioned that an approach towards the large 

 size of the cavity for the reception of the tentacle is perhaps 

 to be found in Ophiomyces frutectosus, Lyman. 



Respecting the under arm-plates, which seem to accord so 

 fully with the homologous plates in Ophiuroidea, it may be 

 remarked that their peculiarity in this instance may be ac- 

 counted for by the abnormal development of the whole orga- 

 nism, and the consequent action of correlation, rather than as 

 the direct outcome of inheritance or functional utility. The 

 original estimate of the ordinal value of these plates, in conse- 



