ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 309 



side, through the other parent. It should be mentioned that, in the case being 

 considered here, intermediate stages were found, where an epaulette was so small 

 that it was difficult to say whether it was present or absent. 



11. The Chaeacters of the Young Urchins and the Inheritance of the same. 



The work described in this section was done in 1912 : the larval characters alone 

 having been investigated in the preceding years. Between the young, just 

 metamorphosed, E. miliaris and E. esculentus there is a definite distil hctioti as 

 described by MacBeide in 1903 (56). After this stage there is no structural 

 difterence between the species until they attain a considerable size. The character 

 described by MacBride was this : At metamorphosis E. esculentus has one terminal 

 tube-foot in each radius. E. miliaris has, at the same stage, besides the five 

 terminal tube-feet, five pairs of lateral tube-feet already developed. Here was 

 apparently a definite character, present in the one form and absent in the other, 

 which would be well suited for the study of heredity, and its inheritance was 

 investigated, as stated above, in 1912. 



At metamorphosis there was considerable variation in the size of the Echini, in the 

 length of the spines, and in the coloration. The more healthy individuals seemed to 

 be larger and to have longer spines. The ground colour varied from a yellow to 

 a salmon-pink, on which were scattered reddish-brown spots. E. miliaris was usually 

 rather pinker and E. esculentus yellower, but there was much individual variation. 



Plate 23, figs. 84-89, give six consecutive stages in the development of E. esculentus, 

 in ventral view. Fig. 84 shows an urchin which had just metamorphosed. The 

 illustration shows that at this stage there is one primary tube-foot in each radius, 

 but as yet no trace of the paired tube-feet. In each interradius is a group of four 

 spines, one dorsal, one ventral, and two lateral. Dorsal to each of the primary tube- 

 feet is a pair of spines with crowned tips. As yet the mouth and anus are both 

 closed. On the dorsal surface are three pedicellarise and several crowned spines. 

 Fig. 85 shows an urchin four days after metamorphosis. The spines have increased 

 in size and serrations are beginning to appear on their surfaces. At the base of each 

 primary tube-foot is a pair of protuberances, which are the rudiments of the first pair 

 of tube-feet of each radius. At the next stage (fig. 86), six days after metamorphosis, 

 these paired tube-feet have become functional, the disc of each being seen in the 

 illustration. The succeeding stage is seen in fig. 87, and shows these paired tube- 

 feet increased in size. The five teeth can be seen surrounding the mouth, and, 

 external to them, the commencement of the oral tube-feet. The groups of four 

 spines in each interradius have increased in size, and the serrations on them have 

 become well marked. It wiU be seen that, in each interradius, the ventral and the 

 two lateral spines have pointed ends, while the dorsal spine has a crowned end. 

 This feature becomes more marked at a later stage, but it is not constant. In some 

 few individuals all four spines of each group have crowned heads, while in others 



