618 DECAPOD CRUSTACEA : — STRUCTURE OF SHELL. 



separate for the protrusion of a large and strong anterior pair of 

 prehensile Limbs provided with an adhesive Sucker and Hooks, 

 and of six pairs of posterior Legs adapted for swimming. This 

 Bivalve Shell, with the Members of both kinds, is subsequently 

 thrown-off ; the animal then attaches itself by its Head, a por- 

 tion of which, in the Barnacle, becomes excessively elongated into 

 the ' Peduncle ' of attachment, whilst in Balanus it expands into 

 a broad Disk of adhesion ; the first Thoracic segment sends back- 

 wards a prolongation which arches over the rest of the body so as 

 completely to enclose it, and of which the exterior layer is consoli- 

 dated into the ' Multivalve ' shell ; whilst from the other Thoracic 

 segments are evolved the six pairs of Cirrhi, from whose peculiar 

 character the name of the group is derived. These are long, 

 slender, many-jointed, tendril-like appendages, fringed with deli- 

 cate filaments covered with Cilia, whose action serves both to bring 

 Food to the Mouth, and to maintain Aerating currents in the 

 water. 



507. Malacostraca. — The chief points of interest to the 

 Microscopist in the more highly-organized forms of Crustacea are 

 furnished by the structure of the Shell, and by the phenomena of 

 Metamorphosis, both which may be best studied in the commonest 

 kinds. — The Shell of the Decapods in its most complete form 

 consists of three strata ; namely, 1, a Horny structureless layer 

 covering the exterior ; 2, an Areolated stratum ; and 3, a lami- 

 nated Tubular substance. The innermost and even the middle 

 layers, however, may be altogether wanting ; thus in the Phyllo- 

 somas, or ' Glass-Crabs,' the envelope is formed by the transparent 

 Horny layer alone ; and in many of the small Crabs belonging to 

 the genus Portuna, the whole substance of the carapace beneath 

 the horny investment presents the Areolated structure. It is in 

 the large thick-shelled Crabs, that we find the three layers most 

 differentiated. Thus in the common Cancer pagurus, we may 

 easily separate the structureless Horny covering after a short 

 maceration in dilute acid ; the Areolated layer, in which the 

 Pigmentary matter of the coloured parts of the shell is chiefly 

 contained, may be easily brought into view by grinding-away from 

 the inner side as flat a piece as can be selected, having first 

 cemented the outer surface to'the Glass slide, and by examining this 

 with a magnifying power of 250 diameters, driving a strong light 

 through it with the Achromatic Condenser ; whilst the Tubular 

 structure of the thick inner layer may be readily demonstrated, 

 by means of Sections parallel and perpendicular to its surface. 

 This structure, which very strongly resembles that of Dentine 

 (§ 546), save that the tubuli do not branch, but remain of the 

 same size through their whole course, may be particularly well 

 seen in the black extremity of the Claw, which (apparently from 

 some difference in the molecular arrangement of the Mineral par- 

 ticles, the Organic structure being precisely the same) is much 



