ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 361 



Fig. 104. — Side view of an urchin of the cross E. esculentus $ X -S. acutus $ . 

 Natural size. Two years old. The general pigmentation underwent great 

 change after the first-year stage shown in fig. 100. None of the hybrid 

 urchins of this cross developed oral spines, in this respect resembling 

 E. acufAis. In general colour, shape of test, and spines, they approached 

 an intermediate type between E. esculentus and E. acatus. 



Fig. 105. — Aboral view of an urchin of the cross E. miUaris ^ X E. acutus $. 

 Four months old. This cross presented two varieties : about half the 

 resulting urchins were of the light pigmented type shown in this figure, 

 while the other half were of a darker pigmented character shown in 

 fig. 107. X 5. 



Fig. 106. — Aboral view of young urchin of the cross E. esculentus '^ xE. miliaris $ . 

 X 6. Four months old. 



Fig. 107. — Aboral view of an urchin of the cross £". miliaris "^ XE. acutus $ . Four 

 months old. The lighter variety of this cross is shown in fig. 105. X 5. 



Plate 25. 



Figs. 108-127. — Photographs of pure-bred and hybrid urchins. 



Fig. 108. — Side view of typical E. esculentus. 



Fig. 109. — Side view of an intermediate specimen approaching E. acutus more closely 



than E. esculentus. 

 Fig. 110. — ^Aboral view of the specimen figured in fig. 109. 

 Fig. 111. — Side view of another intermediate specimen, approaching E. ctcutus more 



closely than E. esculentus. 

 Fig. 112. — Aboral view of fig. 111. 

 Fig. 113. — Side view of a short-spined E. acutus. 

 Fig-. 114. — Aboral view of fig:. 113. 

 Fig. 115. — Side view of a long-spined E. acutiis. 

 Fig. 116. — Aboral view of fig. 115. 

 Fig. 117. — Side view of a typical E. acutus. 

 Fig. 118. — Side view of the test of a typical E. esculentus. 

 Fig. 119.— Test of E. esculentus with unusually prominent primary tubercles {cf. 



E. acutus). 

 Figs. 120 and 121. — Intermediates between E. esculentus and E. acutus. 

 Fig. 122. — Test of typical E. acutus. 

 Fig. 123. — Aboral and oral view of hybrid urchin, two years old, of the cross 



E. esculentus ^ X E. acutus $ . The photograph represents the hybrid 



slightly less than natural size, and is taken from the living animal. In 



the absence of oral spines it resembles the paternal parent. Tliis 



specimen had well-formed genital pores. 



VOL. CCIV.-B. 3 A 



