THE ANNALS 



AXD 



MAGAZINE OF NATURAL HISTORY. 



[FOURTH SERIES.] 



No. 6. JUNE 18G8. 



XLVII. — On Balanus armatus, and a Hybrid between this 

 Species and Balanus improvi,:;us, var. assimilis, Darw. By 

 Dr. Fritz Muller*. 



[Plate XX.] 



In Acasta purpurata, which lives in the bark of an Isis^ as 

 also in Acasta cyathus and sulcata^ which live in sponges, 

 Darwin found that, in the outer branch of the fourth pair of 

 cirri, the anterior margins of some of the inferior joints were 

 armed with strong decurved teeth, by which means, he thought, 

 these joints were converted into jaw-like structures, and became 

 wonderfully well fitted to seize any prey (Darwin, ' Balanidse,' 

 pp. 84 & 311). In no other Cirripede has a similar armature 

 been hitherto detected. 



When I first met with Balanidee imbedded in a sponge, I 

 of course at once looked for this armature, and had the grati- 

 fication of finding both branches of one of the cirri equipped 

 with similar but much more numerous teeth. But on closer 

 examination it appeared, to my great astonishment, that in 

 my species it was not the fourth, but the third pair that bore 

 the teeth, and that the animal was not an Acasta, but a true 

 Balanus with porous walls and a porous base, and scarcely 

 distinguishable, as regarded its shell, from Balanus trigonus, 

 Darwin. 



Occurrence. — This Balanus armatus (as I have called it, on 

 account of the abundant armature of its cirri) lives almost ex- 

 clusively in sponges. I found the first three mutually adhe- 

 rent shells (two of them with the animal still in them) thrown 

 up upon the shore, and rather worn. They appeared not to 

 have been attached to a solid body; and in sheltered parts, 

 especially beneath the deeply excavated base of one specimen, 

 there was some loose sponge-mass, which, from the spicules, 



* Translated by W. S. Dallas, F.L.S. kc, from Wieg-mann's ' Archiv/ 

 1867, pp. 329-356. 



Ann. & Mag. N. Hist. Ser. 4. Vol.'i. 28 



