THE SESSILE BARNACLES. 



9 



not all, of the littoral species attach to ship bottoms; a few forms 

 are known from no other source. Such specimens are often found in 

 museums, labeled merely with the locality where they were taken 

 off of the ship, or picked up where they had dropped or been cleaned 

 off. Such specimens may often be known by the paint or rust 

 adhering to the base, or they may show the grain of the planks. 

 On such smooth surfaces a barnacle is likely to develop a beautifully 

 symmetrical cone, whereas others of the same race, growing on an 

 uneven natural surface, are likely to be irregular, unsymmetrical, 

 or cylindrical. 



Most of the species which develop a conic wall when growing alone 

 become lengthened and cylindric when crowded. The transitions of 

 form are frequently seen in the same group, as in plate 44, figure 2. 

 Very much lengthened forms often result where barnacles grow soli- 

 tary on small objects, as in figures 5-5& of plate 40. 



Like many originally bilateral animals which have become fixed, 

 the sessile cirripedes approximate more or less to a superficial radial 

 symmetry. This is especially marked in the whale and turtle bar- 

 nacles, in which the incidence of external forces is practically equal 



on all sides. 



MATERIAL EXAMINED. 



In the census of species and subspecies of known recent sessile cir- 

 ripedes, given in the first column of the following table, several forms 

 of doubtful specific or subspecific status are admitted. The other 

 columns contain species and subspecies contained in the National 

 Museum. 



