CRUSTACEA. 279 



matter diffused throughout a certain thickness of the calcareous layer. 

 (Plate VI. No. 5.) In the crab and lobster it is very thin, but in the 

 crayfish it occupies in some parts more than one-third of the entire 

 thickness of the shell ; when examined by the microscope; this portion 

 appears to be composed of a large number of very thin laminse, which 

 are indicated by fine lines taking the same direction on the surface of 

 the shell, the number of lines being the greatest in the' oldest speci- 

 mens ; these layers, even in the crayfish, are covered by a thin stratum 

 of very minute hexagonal cellsj without any trace of cell matter in their 

 interior. The corium is the thickest layer of the three, being the one 

 on which the strength of the shell depends, in consequence of the cal- 

 careous material deposited in it. (Plate VI. No. 4.) When a vertical 

 section of the shell of the crab is examined, it is found to be traversed 

 by parallel tubes, like those in the dentine of the human tooth ; these 

 tubes extend from the inner to the outer surface of the shell, and are 

 occasionally covered by wavy lines, probably those of growth, shown in 

 a portion of No. 3, Plate VI. If a horizontal section of the same shell 

 be made, so that the tubes be divided at right angles to their length, 

 the surface will clearly exhibit their open mouths, surrounded by cal- 

 careous matter. In shrimps and very small crabs, the deposition of the 

 calcareous matter takes place in concentric rings, like those of agate ; 

 and occasionally small centres of ossification somewhat similar to No. 

 3, with radiating striae, are to be met with in the former animal. Tf 

 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 preserved. 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, as shown at No. 8, from the outer brown shell of 

 the oyster. The beauty of all these structures is much increased when 

 viewed by polarised light on the selenite stage. 



The sub-class Cirrhipoda includes only a single order. They are 

 all marine animals, which, when mature, attach themselves to rocks or 

 other submarine objects j the common Barnacle (fig. 121), perhaps the 

 best inown example of the order, generally selecting floating objects 

 for this purpose, and frequently covering the bottoms of ships to such 

 an extent as even to impede their progress through the water. The 

 bodies of these animals are soft, and enclosed in a case composed of 

 several calcareous plates ; they formed part of the group of multivahe 

 shells of the older conchologists. The limbs are converted into a tuft 

 of jointed cirri, which can be protruded through an opening in the 



