ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 26? 



and which has been used in these experiments, is Flemingii. E. esciilenlus occurs in 

 Northern European waters only. It is found in the North Sea, up to Iceland and 

 the Faroes, in the Baltic, and down the English Channel as far south as the Bay of 

 Biscay. It is a more shallow-water form than E. acutus, but they are frequently 

 found on the same grounds, as near Plymouth. In the Irish Sea E. acutus is 

 absent, and E. escidentus is found at Port Erin and Millport between the tide- 

 marks. E. acutus extends down into the Atlantic Deep-Sea region, where 

 E. esculentvs is seldom found. 



During the course of this work specimens have been frequently found, which 

 appeared to be intermediate between E. esculent tis and E. acutus var. Flemingii. 

 With some of these examples it was impossible to say to which species they belonged. 

 The average freqixency of such individuals may be judged from the following count 

 of a typical haul from the " Looe-Eddystone " ground, where E. acutus is more 

 frequent than E. esculentus : — 



E. acutus 126 



E. esculentus ... 48 

 Doubtful specimens . 6 



180 



Typical E. esculentus and E. acutus (see Plate 25, figs. 108 and 117) are easily 

 distinguishable from one another by the general form and coloration of the test, and 

 by the shape, length, and distribution of the spines. Definite distinctions between 

 the two are, however, few in number. The chief are as follows : — The number of 

 ridges on the large spines of jE". esculentus is 11 or 12; in E. acutus it varies from 

 14 to 19. In living specimens of ^. esculentus a large number of calcareous plates 

 can be seen on the buccal membranes ; in E. acutus these plates are embedded in the 

 skin. In E. esculentus there is always a large number of small spines on the buccal 

 plates around the mouth ; these spines are absent in E. acutus. Finally, the two 

 forms differ in the size of the eggs. The mature eggs in each of the species of 

 Echinus are very uniform in size. In E. esculentus the average diameter is 0'18 mm., 

 while in E. acutus it varies from 0'13 to 0'14 mm. The specimens which appeared 

 to be intermediate between E. esculentus and E. acutus were intermediate in general 

 shape and coloration and in spine length and distribution, and in the particular 

 distinctions detailed above. Some approached E. esculentus, some E. acutus, while 

 others appeared to stand half-way between the two. The most common type was a- 

 form resembling a short-spined E. acutus, with, however, a few of the small spines 

 around the mouth. One or two only of these doubtful specimens were mature 

 females, and in these the average diameter of the ripe eggs was 0i6 mm., which is 

 intermediate between the O'lS of ^. esculentus and the 0"13-0'I4 o£ E. acutus. 



2 M 2 



