ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 359 



Fig. 82. — E.milimis % X E. acutus $ (the reciprocal of figs. 80 and 81). 28 days. 



0-5 c.c. N/10 NaOH per 200 c.c. sea-water. Note the maternal 



inheritance — absence of posterior epaulettes and presence of green 

 pigment. 



Fig. 83. — The same cross. 28 days. C'.'i c.c. N/10 acetic acid per 200 c.c. sea- 

 water. Maternal characters. 



Plate 23. 

 Figs. 84-97. — Young sea-urchins just after metamorphosis. 



Figs. 84-89. — Six consecutive stages in the development of E. esculenlus from 

 immediately after metamorphosis onwards. 



Fig. 84. — E. esculentus immediately after metamorphosis, showing the single primary 

 tube-foot in each radius. Note also the group of four spines in each 

 interradius and the pair of crowned spines dorsal to each tube-foot. 



Fig. 85. — Sea-urchin of the same species, 4 days after metamorphosis. The rudiments 

 of the first pair of paired tube-feet are apparent at the base of each 

 primary tube-foot. 



Fig. 86. — The same species, 6 days after metamorphosis. The paired tube-feet have 

 developed discs. 



Fig. 87. — The same species. The paired tube-feet have increased in length and the 

 teeth round the mouth are now apparent. 



Figs. 88 and 89. — Succeeding stages of the same species. 



Fig. 90. — E. acutm immediately after metamorphosis. This form is essentially 

 similar to EJ. esculentus at the corresponding stage. There is a primary 

 tube-foot in each radius, with as yet no paired tube-feet. 



Fig. 91. — E. udliaris immediately after metamorphosis. There is in each radius a 

 primary, unpaired tube-foot, and in addition, a pair of small functional 

 paired tube- feet. 



Fig. 92. — E. esculentus ? X E. acutus $ immediately after metamorphosis. Note 

 the single primary tube-foot in each radius. 



Fig. 93. — An abnormal individual of the cross E. acutus ? X E. esculentus $ imme- 

 diately after metamorphosis. Three of the radii had two tube-feet in 

 each, while the remaining two radii developed the usual single primary 

 tube-foot. Note also that one of the interradii developed a group of 

 five spines instead of the usual four. This is very unusual. 



Fig. 94. — E. esculentus $ X -£'. miliaris $ immediately after metamorphosis. Note 

 the single primary tube-foot in each radius, as in the maternal parent. 



Fig. 95. — An unusual example of the cross E. miliaris % X E. esculentus $ , which 

 metamorphosed with four tube-feet in each radius. 



