ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 269 



plates, holds. E. acutus, at any rate when full grown, can have a few of these spines, 

 but they are never numerous as in E. esculentus, and they do not appear at such a 

 young stage. This means that when these spines are found in E. acutus they do not 

 indicate hybridization with E. esculentus. 



Secondly, the suggestion that the two forms are only varieties of one species is ruled 

 out of court by the facts of distribution. On the Swedish coast E. esculentus alone 

 is found, but in the more open sea E. acutus also occurs. Similarly, at Port Erin, in 

 the Irish Sea, and at Millport, in the Clyde, E. esculentus is found without E. acutus. 

 In the Mediterranean, on the other hand, E. acutus is present but not E. esculentus. 

 This distribution alone seems to warrant the separation of the two species. 



Plate 25, figs. 108-117, show a series of specimens from the typical E. esculentus 

 (Plate 25, fig. 108) to the typical E. acutus (Plate 25, fig. 117), with intermediates 

 between. Plate 25, fig. 108, is E. esculentus of the typical form, and has a large 

 number of buccal spines. Plate 25, figs. 109-112, may perhaps be hybrids, but are 

 nearer to E. acutus than to E. esculentus. The secondary aboral spines are, however, 

 more developed than is usually the case in E. acutus, and the primaries are less 

 prominent. The long white spines on the ambitus and tlie few (Plate 25, figs. 100 

 and 110) or no (Plate 25, figs. Ill and 112) buccal spines are E. acutus characters. 

 The shape of the test and the coloration (especially in Plate 25, figs. Ill and 112) are 

 E. esculentus characters. Plate 25, figs. 113 and 114, show an E. acutus with rather 

 short spines and a few of the small buccal spines. Plate 25, figs. 1 1 5 and 1 16, show a 

 long-spined E. acutus of fairly typical form, which had a few buccal spines. Plate 25, 

 fig. 117, shows the typical E. acutus (var. Flemimjii) with no buccal spines. 



Plate 25, figs. 118-122, are a series of tests from E. esculentus (Plate 25, fig. 118) 

 to E. acutus (Plate 25, fig. 122). Plate 25, fig. 118, shows the typical shape and 

 tuberculation of E. esculentus. Plate 25, fig. 119, is a true E. esculentus, but the 

 primary tubercles are rather more prominent than usual. Plate 25, figs. 120 and 121, 

 are so intermediate (especially fig. 121) in general shape and in the arrangement of 

 the tubercles between E. esculentus and E. acutus, that it can scarcely be doubted 

 that they are really hybrids. Plate 25, fig. 122, is a typical test oi E. acutus. 



The foregoing is a precis of the report on the specimens figured in Plate 25, 

 figs. 108-122, which was given us by Dr. Moktensen. It will be seen that 

 E. esculeritus and E. acutus both vary towards one another, but that some individuals 

 are so exactly intermediate that they cannot fairly be said to belong to one species 

 or the other. It seems quite probable that these examples are natural hybrids. This 

 is supported by the fact that hybrids between these two species raised in the 

 laboratory are more or less intermediate in character between both parent species. 



As mentioned above, E. miliaris is typically a shore species. The material used 

 in these experiments was mostly obtained at low spring tides from Wembury Bay 

 and from Cawsand Bay. The urchins are usually found under large boulders and in 

 crevices of the rocks. 



