Dr. F. Mtiller on Balanus armatus. 401 



The coiivpartments of the Shell. — The tubes which penetrate 

 the walls are tolerably wide ; in the uppermost part they are 

 completely filled, without transverse septa. The inner surface 

 of the walls is ribbed longitudinally, usually throughout, but 

 sometimes only below. The radii are externally smooth and 

 shining, with a fine striation in two directions, the one parallel 

 to the septa, the other to the sutural edge. The former is al- 

 ways more distinct; it is not caused by prominence of the 

 septa. In the radii of the rostrum and of the lateral pieces 

 this striation is nearly perpendicular to the walls of the lateral 

 pieces ; in the radii of the carino-lateral pieces it is perpendi- 

 cular to the wall of these pieces. Within, the radii, where 

 they do not lie upon the alai, are often finely ribbed by the 

 projecting septa; these ribs are generally very distinct, but 

 sometimes scarcely perceptible, and sometimes the radii are 

 quite smooth internally. In the septa of the radii, the inter- 

 stices of which are closely filled up to the suture, but often 

 distinguished by their reddish colour from the white septa, I 

 have been unable to detect any distinct denticulation. The 

 sutures of the alaj are smooth. The sheath has a sharp edge 

 projecting freely below. 



Base. — The base is porous ; only in very rare cases it 

 projects beyond the lower margin of the shell. Even in the 

 animals domiciled in sponges the cement-tubes are well deve- 

 loped, whilst in Acasta they were not detected by Darwin. 

 After the base has been treated with acids, they appear as 

 colourless empty tubes. Their ramification differs in dificrent 

 animals, but is exactly the same in the difierent older and 

 younger tubes of the same animal, so that the branches of 

 each younger circle run parallel to those of the older inner 

 ones. Not unfrequently cgecal diverticula occur. At the 

 margin of the base, which is rarely got under the microscope 

 in a good state of preservation, I have seen the cement-tubes 

 dividing into very fine reticulated branchlets, such as Darwin 

 describes and figures in Balanus tintinnabulum (Balanidse, 

 pi. 28. fig.4«). 



Mouth. — The u2-)]jer lip (fig. 10) has three approximated 

 teeth on each side of the central notch. The mandibles 

 have four distinct teeth ; the fifth is sometimes entirely de- 

 ficient, but is usually to be distinguished as a small tubercle 

 above the lower angle of the mandible. On one occasion I 

 found in the same animal the fifth tooth uncommonly dis- 

 tinctly developed on one side, whilst on the other it was en- 



mity, as though a nerve entered the cord ; between the bristles and the 

 chitiuous cords there seems to be a sort of articulation. 



