444 Prof. S. Lovdn on the Structure of the Echinoidea. 



already enlarged and elongated in tlie direction of ambulacrum I. 

 by tlie absorption of tbe vertical plates 1 and 5 at their inner 

 margins ; in tbe interspace small pieces of calcareous network 

 show themselves. The verticals are not yet perforated by the 

 genital pores. Of the eye-plates, I. has already pressed in be- 

 tween ] and 5, and 11. approaches the inner circle. 



Figs. 2-8. Toxopneusfes drobachensis. Ambulacrum III., to show its 

 changes during gi-owth. 1, 2, .3, &c., primary plates and tenta- 

 cular pores; i, 2, 3, &c., large plates; i, 2, 3^ &c., sphseridia; 

 1, 2, S, radiolar tubercles. 



Fiff. 2. Young of 3 millims. The tentacular pores still in nearly unaltered 

 primordial curves. Two sphperidia. 



Fiff. 3, Young of 6 millims. The tentacular pores already distinctly 

 arranged in secondary curves. Four sphaeridia, i, 2, 3, 4. The 

 peristomial plates depressed. The radiolar tubercle 1 in a i 

 much diminished. 



Fiff. 4. Yoimg, somewhat larger. The peristomial plates more depressed ; 

 in « 1 the two primary plates have coalesced. Sphseridium i 

 has disappeared, and 5 has been added. Radiolar tubercle 1 in 

 a very small, that in b diminished. 



Fiff. 5. Young, somewhat larger still. In a, large plates i and 2 have 

 coalesced to form a binary peristomial plate ; in b the sutures of 

 the primary plates have disappeared. Radiolar tubercle 1 in a i 

 lost, in J 1 diminished. 



Fiff. 6. Young of 11 millims. diameter. In this, as in the following, 

 large plate 3 in a has only three primary plates. Large plates 

 1 and 2 coalesced into a binary plate, also in b. Radiolar tu- 

 bercle 1 has disappeared both in a and b. 



Fiff. 7. Young individual of 15 millims. The binary large plates 1+2 

 in both rows depressed. Sphperidium 2 has disappeared, and 

 6 and 7 have been added. Radiolar tubercle 2 dimmished. 



Fiff. 8. Adult individual, of 52 millims. diameter. Large plates i, 2, and 

 3 in both rows have coalesced to form ternary peristomial plates. 

 One sphperidium, 8, added. In b there is still, apparently, a 

 residue of radiolar tubercle 1. 



Fiff. 9. Stoma of a very yoimg Toxo/meustes drobachensis of 2 millims. 

 diameter. Of the free plates in the buccal membrane those 

 which belong to I. a, II. a, III. b, IV. «, V. b, have not yet ac- 

 quired pores. One sphperidium in each ambulacrum in I. b-\. a. 



Fiff. 10. Young of some northern Echinide, 0'6 millim. in diameter, from 

 the ventral side. The pigment-spots are omitted, and the 

 radioli only partially represented. No remains of the larva ; no 

 trace of jaws; no mouth or anus. Five large primordial tenta- 

 cles, which, according to Krohn, soon disappear. Within these 

 five pairs of smaller permanent tentacles, each of which stands 

 over a disk of calcareous network, the first primary plates in 

 the ambulacra. Outside of and between the pairs of these there 

 are other disks, probably the foundations of the interradia. • From 

 an individual preserved in spirit. 



Fiff. 11. One of the large primordial tentacles vsdth its disk, and in the 

 latter a ring of calcareous net. In the wall of the tube longitu- 

 dinal and transverse muscular fibres. 



Fiff. 12. One of the smaller permanent tentacles in the young animal, 

 fig. 10, with its disk and a portion of the calcareous net which 

 lies beneath its base. 



Fig. 13. The oblong, externally pointed opening in the calcareous network, 



V the pore, over which the tentacle stands. 



