i AG 47h my Sera es erat de fa Z a8 ag 8G , aes ore og Uta 
een Me “ko a ! 
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 Balani than to Acasta; B. armatus and spongicola 
_ 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. 
If. 
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 balanotdes, although there 
were well-developed larve in their shells (Balanide, 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 
which I have recently made have confirmed me in this doubt. 
It is well known that the Balan? are very sensitive to lightT, 
so that they immediately retract their cirri and close the oper- 
* Fritz Miiller, ‘Fiir Darwin,’ p. 20. 
+ The sensitiveness of the Balani to luminous impressions is not de- 
pendent on the eyes discovered by Leidy. I had taken a large Balanus 
tentinnabulum 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. tintinnabulum 
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 easy to detect even in small specimens of B. tintinnabulum. 
