KOXGL. SV. VET. AKADEMIENS HANDL. BAND 19. N:0 7. 41 



cosa Lamk. of 160 ram., and to another of 0,iö mm., from an Echinocardium flave- 

 scens O. F. M., measuring 28 mm., the relation between the length of the spherid and 

 that of the whole skeleton is 



in Meoma ventricosa as 0,16 to 100, 



in Echinocardium flavescens as 0,5 3 to 100, 

 in Pourtalesia Jeifreysi as 0,7 6 to 100. 



The increased volume of each singie spherid appears in some degree to make up 

 for their reduced number, and its efficiency to depend, in some raeasure, on the ex- 

 tent of its ciliated surface. 



The fragments given me by Sir Wyville Thomson enabled me to examine these 

 curious organs in three more species of Pourtalesia: P. laguncula, P. carinata and P. 

 ceratopyga of Al. Agassiz, Pl. VI, fig. 37, 38, 40, 41, 44; VII, 47, 48, 49, 50. In 

 these, as in P. Jeffreysi, the entire set is brought together on the sub-labial parts of 

 the first plates of the four ambulacra I, V, II, IV, there being none on the frontal, 

 III. P. laguncula has four spherids, one on each of the ambulacrals I a, I b, V a, V b, 

 Pl. VI, jig. 37, 38, 40, 41. They are pear-shaped, more lengthened in proportion than 

 those of P. Jeffreysi, and placed in slight depressions close behind the first pedicels. 

 In P. carinata, Pl. VI, fig. 44; VII, 47, their form is the same. I counted nine in 

 all, one on each of the five first plates of I 6, V a, II a, IV b, IV a, and two on 1 a 

 and V h, but it is probable that some more were löst. The pedicellar portion of each 

 plate is raised, and produced aborally into a projecting point, to which the first spherid 

 is attached. In P. ceratopyga, Pl. VII, fig. 48, 49, 50, they are likewise pear- 

 shaped, and still more numerous. I counted twenty two of them, twelve on the 

 right, ten on the left, but there had no doubt been twenty four. The plates I a 1, 

 1 b 1, Y b 1, Y a 1, had four spherids each, \\ a 1 and II 6 ^ had each two, and IV 

 b 1 also two, but on IV a 1 none could be detected, probably from its two spherids 

 having being löst. 



Thus the spherids of the Pourtalesiadte from one species to another seem to differ 

 more widely in number than those of the other Echinoids. But, whether few or many, 

 they are always found stationed exclusively on a restricted part of the sub-labial region, 

 formed by the ambulacra I, II, V and IV, and absent on the pai't of the peristome 

 that is elevated above it, ambulacrum III, while in Echinoids generally, in which the 

 whole of the peristomal region is ventral, they are distributed all around, on all 

 the five ambulacra, and missing only higher up, on the sides and on the back. Of 

 whatever nature, therefore, the special changes in the surrounding water may be, that 

 their ciliated epithelium has to watch for, these changes seem to be of essential mo- 

 ment to the animal, solely when they take place iu the vicinity of the mouth, or 

 between the under surface and the ground, on which the animal has to find its food. 



The pedicels of the Echinoidea, of paramount importance as organs of touch, 

 of locomotion and prehension, and, in some forras, of respiration, are, however, easily 



K. Vet. Akad. Handl. Band 19. N:o 7. 6 



