Dr. F. Miiller on a Hybrid Balanus. 405 



no means nearly related, that this armature is developed into 

 large curved teeth, intimates that there is a connexion between 

 the peculiar armature and the peculiar domicile; and it is not a 

 far-fetched supposition that the teeth serve to tear in pieces 

 and remove the rapidly growing sponge-mass which threatens 

 to grow over the aperture of the shell. It is a remarkable 

 circumstance that in Acasta the teeth stand on the outer 

 branches of the fourth, and in B. armatus on the branches of 

 the third pair of feet. This circumstance might be adduced in 

 favour of the Darwinian view of the origin of species, in the 

 same way as the different structure of the posterior entrance 

 to the branchial cavity in the different air-breathing crabs*. 

 Balanus armatus is much more nearly allied to other, not 

 spongicolar i?a/aM/ than to Acasta] B. armatus and sjyongicola 

 on the one hand, and the species of Acasta on the other, can- 

 not consequently have inherited the habit of domiciling them- 

 selves in sponges from a common ancestor. Contrivances 

 which stand in relation to this peculiar dwelling-place must 

 therefore have been produced independently in each case ; and 

 therefore it cannot appear strange that we find them developed 

 on different parts of the body in Balanus armatus and in 

 Acasta. 



II. 



Until recently the Balani passed universally as self-impreg- 

 nating hermaphrodites. But that self-impregnation does not 

 take place in all cases was proved by a remarkable observation 

 of Darwin's, who found the penis rudimentary and imperforate 

 in several individuals of Balanus halanoides, although there 

 Avere well-developed larvae in their shells (Balanid^, p. 101). 

 To me it has long been doubtful whether self-impregnation is 

 really the general rule. For what purpose should the length 

 of the penis be often three times the diameter of the shell, if it 

 has nothing to seek outside the latter ? Some observations 

 Avhich I have recently made have confirmed me in this doubt. 



It is well known that the Balani arc very sensitive to light f, 

 so that they immediately retract their cirri and close the oper- 



• Fritz Miiller, ' Fiir Darwin,' p. 20. 



t The sensitiveness of the Balani to luminous impressions is not de- 

 pendent on the eyes discovered by Leidy. I had taken a large Balanus 

 tintinnahidum living out of its shell, and separated it from the operculum, 

 with which the eyes remained in connexion. It lay in a saucer of water, 

 with its cirri half unrolled. As often as the shadow of the hand fell upon 

 it, it rolled up the cirri with a sudden movement. In B. tintinnahulum 

 the eyes are very distinct : in B. armatus I have not yet found them ; 

 and this is not due to the smaller size of the latter species, as they are 

 very f asy to detect even in small specimens of B. tintinnahidum. 



