142 DISCOPHOR^. Part III. 



lower SLU'face of the Scutellicl^, and other Echinoderms. Small pores, like punctures, 

 are visible at the angles of these folds, and may be a means of connnunication 

 between the chymiferous system and the surrounding medium, akin to the minute 

 pores of the lower floor of the Clypeastroids. Fi(j. 3 of PI. XIII. represents a young, 

 showing fewer ramifications and a much less comiilicated structure of the arms near 

 the centre, than in the adult; though even at this age their margins are already 

 closed, and it is evident that there are eight arms resulting from the division of 

 four main stems, the arms remaining generically closed together (/ f-\ though they 

 separate occasionally {1} f, f f), to a greater or less degree. The other figures 

 exhibit structural details to which I shall allude presently. 



The characteristic combination of spheromeres which distinguishes the family of 

 the Polyclonidae, consists in a central sub-quadrangular cavity (PI. XIII. Fiff. 4), 

 formed by the combination of four spheromeres, in the ambulacral rays of which 

 are, morphologically considered, only four arms, extending in the radial prolongation 

 of the four rounded corners of the main cavity, but dividing at once into two 

 symmetrical branches, Fif/s. 2 and 3 ; while, in the interambulacral rays, there are 

 four genital pouches, alternating with two and two of the arms, and occupying 

 the middle of the sides of the main cavity, through the wall of wliich open the 

 holes leading from the outside into these pouches {Fi(/. 4, oc oc), though the pouches 

 themselves (o os os') are closed ; so that there is no possible communication between 

 the main cavity into which the genital pouches project and the sacs below them, 

 opening outward between the arms. Another very unusual combination is notice- 

 able in the position of the eyes, of which there are twelve, four in the radial 

 prolongation of the axis of the arms, and two corresponding to each of the four 

 sides of the main cavity; no one of these, however, being in the radial prolon- 

 gation of the centre of the genital pouches. In the true Rhizostomidas there are 

 only eight eyes, four in the radial prolongation of the arms, and four in the radial 

 prolongation of the genital pouches. Here we have two eyes to each genital pouch, 

 neither of them in its radial jirolongation, but both, on the contrary, occupying a 

 lateral jjosition with reference to the genital pouches, though, with reference to the 

 ambulacral eyes, they are placed at equal distances in the margin of the disk. 



The system of radiating chymiferous tubes presents corresponding differences 

 when compared to that of Ehizostoma. In the latter genus there arises one main 

 chymiferous tube, in the radial j^rolongation of each of the four arms and of each 

 of the four genital pouches, extending in the direction of the eight eyes, while 

 eight others alternate with those of the eyes. These sixteen main branches 

 extend for half their course without giving off any branches; while in their peri- 

 pheric course they foi-m innumerable small anastomoses, connected with each other 

 and with the main branches by transverse branches and by a few large meshes 



