STRONUYLOCENTROTUS DROBACHIENSIS. 281 



tachment to the teeth. The ten perforated plates carrying tubercles, which 

 in the adult are close to the base of the teeth, are found to be close to the 

 test. In a somewhat older specimen, measuring 3.5 mm - in diameter and 

 having nine : ten plates, there are traces of four additional minute plates in 

 the anal system {PL IX. f. 6). There is no change in the arrangement of the 

 pores or of the tubercles; when seen from the lower side, the actinal opening 

 occupies a much smaller proportion of the test {PL IX. f. ;). The tentacular 

 plates are nearer the centre, and the ambulacral and interambulacral spaces 

 on the periphery of the actinostome are equal, the latter having been the 

 larger in the younger stages. In a specimen somewhat older, measuring 

 5.1 mm in diameter, the notches of the actinostome are more marked, the 

 opening reduced still further in proportion {Pl.IX.f. 10); and in specimens 

 of SA™™- in diameter the actinal opening has about the proportions of the 

 adult {PL IX. f. 12), the ambulacral area is broader than the interambu- 

 lacral at the periphery, the notches are about as they were in the preceding 

 stage. In the specimen measuring h.V™* in diameter, we find the first trace 

 of the open arcs of pores towards the ambitus {PL IX. f. 9), while the upper 

 tubercles are still surrounded by closed vertical arcs {PL IX. f. S). It seems 

 impossible to account for the subsequent arrangement of the pores, except 

 on the supposition that the plates forming the poriferous zone grow indepen- 

 dently of the ambulacral plates, and that they are pushed along down 

 towards the ambitus, forming open arcs of four or five or even a larger 

 number of pairs of pores. 



In all these stages {PL IX. f. 3—1%), the test was quite depressed ; it is only 

 when it attains a diameter of 8.4 mm , that the depression is not marked, and 

 the young Sea-urchin assumes the general outline and characters of the adult, 

 as is shown by the greater number of plates of the anal system and the 

 regularly open arcs of pores, as well as the appearance of the secondary 

 rows of tubercles.* 



* The same general changes fake place in S. lividuo as in S Drobachiensis. In very small specimens, 

 2.5 n,m - in diameter, and having four : five coronal plates, we find four pairs of pores for each ambulacral 

 plate arranged in an almost vertical row and but slightly curved. The same is the case in specimens 

 measuring 3.C mm - in diameter, with six : five tubercles when the curve is somewhat more decided. In speci- 

 mens having seven : eight plates, and measuring 6.3 mm - and 8.4 mm - in d'ameter, we find still the same vertical 

 arrangement of the pores. The arcs do not open till the Sea-urchin has attained a diameter of 10 n ""-, 

 when we find in one plate an arc of five pairs of pores, anil in specimens measuring 12.7 mm -, two plates 

 with open arcs of five pairs of pores. Young S. lividus are much more flattened than the young of S. 

 Drobachiensis, being generally mote turban-shaped, while the latter are always very slightly conical. The 



