66 
side-teeth), the spicules, and perhaps also the apical system (the 
ocular plates being partly in contact with the anal area) — but 
there is no indication of any of these characters. 
The conclusion seems then inevitable that this specimen repre- 
sents a hybrid between Echinus esculentus and Parechinus miliaris. 
There is, to be sure, one other explanation possible — that it 
may represent a case of discontinuous variation or a mutation of 
Echinus esculentus. But this is, as concerns thé Echinoderms at 
least, a quite unknown field as yet. I can give no definite proof, 
that the present case is not really to be thus explained; but, so 
far as I can see, evidence goes towards declaring the specimen to 
be a hybrid. The final solution of the question can scarcely be 
given before we have obtained fullgrown specimens of artificial hybrids 
between these two species. 
It should still be pointed out that both Parechinus miliaris 
and Echinus esculentus occur only in the northern part of the 
Sound, the influence of the brackish Baltic waters making the 
salinity too low for their thriving farther down in the Souud. This 
fact, that the two species in question are thus limited here in 
their distribution towards the Baltic through the lowering of the 
salinity of the sea water may well be worth recalling in this 
connection. May it not be supposed that the low salinity has 
made the egg less resistant towards the spermatozoon of the other 
species? — If this is really the case, one should expect hybrids 
to be comparatively common on such places. This part of the 
question, at least, can easily be solved by future investigations on 
the locality. 
25.—1.—1911. 
