8 



BULLETIN 93, UNITED STATES NATIONAL MUSEUM. 



There are sometimes bunches of short spines on the anterior face 

 between the spines of the pairs. In the species figured (fig. 6) the 

 posterior ramus of the first cirrus and both rami of the second are 

 formed of what Darwin termed "protuberant segments." 



In many species of Balanus there are minute "spinules" variously 

 arranged on the outer faces of the segments of some of the cirri, par- 

 ticularly the third and fourth, as shown in figure 8 and many others. 

 Sometimes the spinules are enlarged on the anterior margin, of the 

 cirrus, forming "teeth," as in Acasta (fig. 80) and some Balani. 



In the evolution of barnacles the cirri have been successively modi- 

 fied from before backward. In the least modified forms the second 

 cirrus, or only its anterior ramus, has assumed the form and chseto- 

 taxy of the first cirrus. In more advanced forms the second and 

 third cirri are so modified. In a few, the fourth cirrus also shows 



FIG. 6. CtBRI OF THE EIGHT SIDE OF BALANUS KOSTRATUS ALASKENSIS, FROM IvODIAK, X3. CAT. NO. 

 3415. THE PENIS IS SEEN AT THE BASE OF THE SIXTH CIRRUS. THE SPINES ARE NOT SHOWN IN THIS 

 PHOTOGRAPHIC FIGURE. 



some modification. The modification of the cirri is therefore one of 

 the indices to the evolutionary rank of a species. 



On account of the fact that a barnacle grows at the base and sutures, 

 and not at the orifice, it receives the impression of the supporting 

 surface, and often reproduces its sculptural features. A specimen 

 growing on a scallop shell may bear, vicariously, the ribs of the 

 shell, as in plate 33, figure 2a. Exactly the same effect, from the 

 same cause, is sometimes seen in sedentary, limpetlike mollusks. 

 This vicarious sculpture may be superimposed upon that proper to 

 the barnacle, or the conflict of the two patterns may result in neither 

 being distinctly expressed. Very often the sculpture of the support- 

 ing surface seems to have no effect whatever on that of the barnacle, 

 even when the latter has a smooth surface. 



The shapes of barnacles are often controlled by the size and shape 

 of the supporting object, but they show little if any choice between 

 such solid objects as occur in their vicinity. Many, but apparently 



