CRUSTACEA. 



431 



strise, are met with in the Shrimp. If the calcareous 

 portion of the shell be steeped in hydrochloric acid, a 

 distinct animal structure or basis is left behind, and 

 the characters of the part will be very accurately pre- 

 served. The calcareous matter, like that of bone, generally 

 presents a more or less granular appearance as at No. 4, 

 and so angular in figure as to resemble certain forms of 

 rhomboidal crystals, shown at No. 2, from the outer brown 

 shell of the Oyster. The beauty of all these structures is 

 much increased if viewed with polarised light on a selenite 

 stage. 



Cirrhopoda or Cirripedia, when mature, attach them- 

 selves to rocks and other objects ; the Barnacle (fig. 214) 

 and Acorn-shell are the best 

 known examples of this order; 

 they generally select floating ob- 

 jects to dwell upon; and bottoms 

 of ships have been covered by 

 them to such an extent as even to 

 impede their progress through the 

 water. The soft bodies of these 

 animals are enclosed in a case com- 

 posed of many calcareous plates ; 

 from this circumstance they were 

 grouped with the multivalve shells 

 of the older conchologists. The 

 limbs are converted into tufts 

 of jointed cirri, and protrude 

 through an opening in the man- 

 tle Which lines the interior of 

 J.T T_ n mu i 



the Shell. I he Cirri, twelve in 



number, are covered with cilia, 

 which, when the animal is alive, are in continual motion. 

 The intestinal canal is complete, and the nervous system 

 exhibits the usual series of ganglia, which is characteristic 

 of the articulate type. The head is marked only by the 

 position of the mouth, and is armed with a pair of jaws, if 

 we may so term the shells. 



Balanidce, " Sea-acorns," a sessile species, whose curious 

 little habitations may constantly be met with upon the 

 rocks of the sea -shore, and not unfrequently upon many 



Fig. 214. 

 Young fry of the Oyster, a por- 

 tion of them with cilia pro- 

 truded. 2, Body and cirri of 

 Barnacles. 



