ir> 



ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 335 



edge of the plate. The spines, as in the ambulacra, are very short, being not more 

 than 1 mm. in length. The number of interambulacral plates is seven or eight. 



The apical system, which is formed in the shape of a cone, appears to have the 

 genital and ocular plates of the usual shape. All the ocular plates are excluded from 

 the periproct. The madreporite is very conspicuous and somewhat larger than the 

 other genital plates. There is a single small tubercle at the upper edge of the 

 genital plates. Genital pores cannot be seen. The anal area is very much raised, 

 the anal opening being on top. 



The anal plates are indistinct, but the suranal plate seems to be larger than the 

 adjoining plates. 



The peristome is very large, being two-thirds the diameter of the test. It 

 appears to be quite naked. 



The pedicellaricB are very few in number, only one or two triphyllous and a single 

 ophicephalous could be observed. None are to be seen on the buccal plates. 



The colour- is a reddish-brown in the interambulacral areas and somewhat ligliter 

 in the ambulacral. A dark spot is present on each genital plate, except the 

 madreporite. The ocular plates are slightly greenish in colour, the anal area being 

 dark brown. The spines are light brown. On some of the plates there is a small 

 brown knob in the skin near the base. The tube-feet are practicrdly colourless. 



The very slight development of the spines on the aboral surface of this hybrid 

 recalls the condition found in some small Arbaciids. Its manifest abnormality 

 renders useless its comparison with the parent forms. 



E. miliaris ? X E. acutus $ (Cf. Plate 24, tigs. 103, 105, 107, and Plate 25, fig. 127)— 

 A number of urchins of this cross (35-45) survived and attained considerable size 

 by the end of the first year. At the time they were about five or six months 

 old their pigmentation was quite difterent from that shown at present. In the early 

 stages they presented two well-marked varieties. One was a light and the other 

 was a dark pigmented type. These have been figured in Plate 24, figs. 105 and 107. 

 As they grew older these two varieties became more or less indistinguishable, and their 

 present appearance is something like that drawn in fig. 103. They were derived 

 from the two fertilizations B and K. 



B = E. miliaris % X E. acutus ? , fertilized 15.5.12. — 



Plutei all paternal. 



A flourishing culture, which coinmenced to metamorphose 5.7.12. 



Inheritance of tube-feet character : — ■ 



Paternal 79 urchins 



Intermediate [i.e. with t7-aces of the first paired tube-feet) 5 „ 

 Maternal 1 ,, 



Total 85 „ 



