128 THE VOYAGE OF H.M.S. CHALLENGER. 



directed upward and slightly inward, are placed round this semicircular margin and form 

 an elegant fringe or comb, which, closes over, as it were, upon the series of lamellae. 



On examining this organ microscopically, it is found that each line or lamella is made 

 up of a series of small lamellae, which are placed end to end together, and thus form an 

 apparently continuous line. Each component part or small integral lamella stands 

 upright upon its own rounded scale-like base ; and the lamellar plates are made up of 

 a single series of delicate rods united by irregular dissepiments, the whole structure being 

 covered with a membrane, which appears to have been furnished with vibratile cilia. The 

 scale-like plate which forms the basal portion is directly superposed upon the surface of 

 the marginal plate, the parts occupied by the cribriform organ being slightly hollowed 

 out for its reception. The outermost lines (pseudo-lamellae) are composed of thicker 

 individual lamellae than any of the others, and these integral lamellae stand wider apart 

 and resemble flattened spinelets, each built up of several series of rods. On the upper 

 portion of each line transition can be traced from the delicate lamellae, above described, 

 to the simple rounded cylindrical spinelets of the abactinal membrane. 



Judging from the position and character of this organ, as well as from its relation 

 to the abactinal area, it is not improbable that it acts as a percolator ; and in such 

 a case it might perhaps be looked upon as the homologue of the armature of minute 

 ciliary spines which borders the vertical furrows that run between the consecutive mar- 

 ginal plates in Astropecten and other forms. These fringes of delicate miliary spinelets 

 in Astropecten were regarded by Alex. Agassiz 1 as probably the representatives in 

 Asteroids of the specially locabsed bands of delicate ciliary spinelets known as "fascioles," 

 which are present in many of the genera of irregular Echinoids. 



In species which have more than one cribriform organ in each interbrachial arc (Plates 

 XXL, XXII., XXIII., XXIV., XXV., XXVI. and XXIX.), the additional ones occur on the 

 vertical sutures immediately succeeding on each side of the median line, and are identical 

 with the median organ just described. No case of irregularity or intermission occurs in 

 any of the specimens I have examined. The number of cribriform organs present in each 

 arc appears to be always constant in a species ; and species exist which possess one, 

 three, five, seven, nine, or even fourteen of the organs respectively. The organ varies 

 in the different species as regards its breadth, the number of vertical parallel lines or 

 pseudo-lamellae which compose it, and the character of the integral calcareous bodies, of 

 which these latter are formed (Plates XXVII. and XXVIII.) In Porcellanaster the com- 

 ponent parts are more or less lamellar in form, as described above, whilst in the allied 

 genera Hyphalaster, Styracaster, Tlwracaster, the corresponding elements are papilliform 

 (Plates XXVIII. and XXIX.) 



TJie Segmental Pits and Papillse. — These are peculiar structures situated on the 

 adambulacral plates and the mouth-plates (Plate XXVII. figs. 2, 3, 6, 7). They consist 

 1 North American Starfishes, Mem. Mus. Comp. Zobl., Harvard, 1877, vol. v. No. 1, p. 119. 



