ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 



333 



E. escidentus ? X E. acutus ^ , fertilized 15.5.12. Commenced to metamorphose 



26.7.12. 



4 urchins metamorphosed. 



Diameter of the test, 2 "5 cm. Height, 1*5 cm. 



The ambulacral plates in nearly all cases have a primary spine, but towards the 

 apical area the irregularity typical of the full-grown individuals of the two parental 

 species begins to appear. Some of the plates here have only a secondary spine 

 placed on the outer part of the plate outside the primary series of spines. A secondary 

 spine appears on the sixth to the eighth plate from above situated near the median 

 edge, but it does not occur regularly on all the plates from this point downwards. 



The interambulacral plates have all a primary spine. On the fourth to the fifth 

 plate from above, there is a secondary spine at the upper inner edge of the plate, and 

 this is repeated on each successive plate down to and below the ambitus. On the 

 seventh plate there is also one on the outer upper edge, which is likewise present on 

 the successive plates. The length of the spines at the ambitus is about 1 cm. 



The apical plates, the limits of which cannot be made out distinctly, carry some 

 spines. The genital plates each carry two rather large spines. The ocular plates 

 each a small one. The genital pores would seem to have been formed. The anal 

 plates are few in number and rather large. The suranal plate is not distinctly larger 

 than the adjoining plates. There are no spines on the anal plates. 



In the living animal the plates of the buccal membrane cannot be observed through 

 the skin. There are no spines on the buccal plates. 



The colour of the test with the apical plates is a dark brownish red. The median 

 line of the interambulacral areas is a somewhat lighter colour, and the pore areas are 

 almost white. Towards the ambitus the colour gradually diminishes in intensity. 

 The oral side is nearly white. The anal area is lighter coloured than the apical 

 plates. The spines at the ambitus are a yellowish green at the base, the colour 

 gradually diminishing towards the tip. The tip itself is almost white. The spines 



