60 PaOF. p. M. DUNCAN AND MB. W. P. Sl/ADEN ON 



curve up to the outer eud of the plate. Consequent on the 

 forms of the two small demi poriferous plates above described, 

 the intermediate portion or prolongation of the large primary 

 poriferous plate is inclined at an angle adorallj, and its outer 

 t nd is slightly enlarged (see fig. 1). The enlargements of the 

 three plates above referred to correspond to the position of the 

 relatively large, suboval or palette-shaped peripodia. 



At a few plates above the ambitus the three peripodia with 

 their pairs of pores form an almost vertical, and very slightly 

 curved, arc ; at the ambitus the curvature of the arc is more 

 definite ; and below that position the peripodium of the adoral 

 demi-plate is seen to have assumed a much more internal posi- 

 tion in relation to the other peripodia of the compound ambu- 

 lacral plate to which it belongs. (Kg- 1 represents the fifth 

 and sixth tubercle-bearing plates, counting from the peristome.) 

 This apparent moving inwards of the adoral peripodium, away 

 from the outer end of the plate, is continued in each succeeding 

 ambulacral plate down to the peristome. Below the ambitus the 

 peripodium of the aboral demi-plate also moves slightly inwards, 

 away from the outer margin of the ambulacral plate, but to a 

 much less degree. The peripodia as they approach the mouth 

 maintain with but very slight diminution their uniform size; 

 the plates, however, upon which they are borne, in the four or 

 five ambulacral plates next to the peristome, decrease successively 

 in depth and become mere band-like strips. Consequent on this 

 and the inward movement of the pores above referred to, the 

 crowded and almost transverse arrangement of the peripodia 

 near the peristome is produced. When a suitable preparation is 

 examined under the microscope, it is found that although the 

 two or three ambulacral plates next the peristome have more or 

 less lost their superficial individuality, the constituent poriferous 

 plates are still distinguishable as independent band-like strips 

 occupying their original relative position, and^that the peripodia 

 are expanded, and poured over superficially as it were, occupying 

 an area whose diameter is much greater than the mean depth of 

 the plate to which they belong. Figs. 4 and 5 illustrate these 

 observations in another species ; and a comparison of the two 

 will at once indicate how the crowded mass of peripodia near 

 the peristome may be reduced to order, in conformity with their 

 disposition at the ambitus. It will be readily observed that the 

 more or lees regular arrangement in obliquely transverse lines 



