334 MESSES. C. SHEARER, W. DE MORGAN, AND H. M. FUCHS 



near the apical area are more brilliantly coloured, the colour being a reddish yellow 

 at the base, which changes to a greenish tint towards the tip. The tip itself is 

 white. The secondary spines are white or a slightly yellowisli green at the base. The 

 tube-feet are colourless. 



The pedicellarice do not present any marked features of interest, which is not to be 

 wondered at considering that they are almost identical in structure in both parent 

 species. 



On account of the similarity of the two parental species of this cross it is almost 

 impossible to say whether the hybrid urchin in its present size is maternal or 

 paternal. In general appearance it seems to be clearly intermediate between 

 E. esculentus and E. acutus. 



E. miliaris ? X -£". esculentus $ — 



One small abnormal urchin of this cross alone survived at the end of the first 

 year. This had grown slowly after metamorphosis, and the test had assumed a 

 somewhat unusual conical shape. 



This urchin was derived from the only fertilization of E. miliaris eggs with 

 E. esculentus sperm in 1912 which gave a high percentage of fertilization (see p. 307). 

 In the culture 10 of the plutei developed posterior epaulettes and no green pigment 

 (paternal), and 25 developed no posterior epaulettes but green pigment. The latter 

 were the only E. miliaris ? hybrids in 1912 to develop the maternal characters. 



The paternal and maternal plutei were separated from one another, in separate 

 jars. Only the 2oaternal plutei metamorphosed. These constituted Culture H. 



Metamorphosis : — 



25.7.12. Two urchins with $ tube-foot character. 



16.8.12. One urchin with $ tube-foot character. 



One urchin with 4 tube-feet in each radius. 



21.8.12. One urchin with 4 tube-feet in each radius. 



25.8.12. One urchin with 1, 1, 1, 2, 3 tube-feet respectively in the five radii. 

 Diameter of test, 4 mm. Height, 4 '5 mm. 



The ambulacral plates. — Each plate has one primary tubercle ; no secondary 

 tubercles (spines) are present. The spines are very small and distinct only towards 

 the ambitus ; they appear on the fifth and sixth plate. In one ambulacrum they 

 appear from the second to the third and are repeated on each successive plate down- 

 wards. The number of ambulacral plates is eight to nine. 



The i7iterambidacral plates likewise all carry a primary tubercle. The spines 

 appear in one area only from the fifth plate downwards. In the other areas they 

 appear on the third. Again in another area they are developed on all the upper 

 plates, but are wanting on a pair of plates at the ambitus from which they have been 

 probably lost. A secondary spine is developed at the upper outer corner on some 

 plates at the ambitus. In a few cases, there is also a secondary spine at the inner 



