r26 



I.UIIRYOLOOY. 



as we might wish. The figures of Miiller are not drawn in such a way as to 

 discriminate between the parts which belong to the ambulacral and to the 

 interambulacral spaces. This is particularly evident in his figures drawn from 

 the mouth side, where we frequently find such a number of tentacles repre- 

 sented as make the development of the different ambulacra unequal. The 

 same is the ease with the spines. Any one who will take the trouble to 

 compare the figures of young sea-urchins of Plates IV. and VII. of his first 

 Memoir, Plate VII. fourth Memoir, and Plate IV. seventh Memoir, with the 

 figures given here (PI. IX. f. / ; PI. X.f. J-4), will see that, although they 



Fig. 59. 



Fir.. 60. 



agree in their general characters, yet. it is impossible to place the different 

 spines or tentacles in such positions that they can lie divided into ambulacral 

 and interambulacral areas, while this is easily done with the figures I have 

 given. We must remember, however, that most of Midler's figures are Cly- 

 peastroids and Spatangoids. which may make it difficult, if not impossible, to 

 divide the young sea-urchin into ambulacral and interambulacral areas, in 

 suborders where we have nothing like regular vertical rows to guide us, as 

 in our common sea-urchin. One great difference, however, will strike us at 

 once ; it is, that what Miiller has called the anus I have in my figures called 



