METAZOA CRUSTACEA. 333 



seen in No. 817, Balanus hameri, where they occur so 

 large in size that they can be easily studied. 



Variation in the color and ornamentation of the shell 

 is well shown by Balanus t'mtinnabulum Linn. (No. 818). 

 In three specimens the shell is smooth, while the speci- 

 men on the right is ridged. 



Sometimes barnacles attach themselves to a whale and 

 become imbedded in its skin. This is the case with 

 Coronula diadema Blainv. (No. 819). The usual conical 

 shell may become modified into a tube, as in Tubicinella 

 balaenarum (No. 820). It is made of eight vertical sec- 

 tions and is marked by circular ridges and perpendicular 

 striae. At the top the four valves are open in No. 820, 

 as when the thoracic appendages or food-catchers are put 

 out. 



The barnacle Concho derma aurita Olf. (No. 821) is a 

 peculiarly modified form. The young, represented by 

 four specimens in the bottom of the bottle, have a short 

 peduncle and a body that is similar in shape to that of 

 Lepas (No. 815), though it is never covered by a shell. 

 In the two youngest specimens this body is colorless, but 

 in the two older ones it is distinctly banded with brown. 

 As Conchoderma grows older the peduncle becomes long 

 and the body less compressed, while both are dark colored. 

 From the upper side of the body grow out two tubular 

 organs which are very conspicuous in the adult (No. 821). 

 Their function is not known with certainty, though Kings- 

 ley thinks they may be respiratory organs. 



MALACOSTRACA. 



An ancestral form of the Malacostraca may be repre- 

 sented by Palaeocaris typus M. & W. (PI. 822, fig. i, 

 enlarged three diameters). The body is long and except- 

 ing the head, is made up of distinct and similar segments 

 which are uncovered by a carapace. The two pairs of 



