214 MANUAL OF THE MOLLUSCA. 



to the other, nearly vertically, and regularly, the distance and size of 

 the perforations varying with the species. Their external orifices are 

 trumpet-shaped, the inner often very small; sometimes they bifurcate 

 towards the exterior, and in Crania they become arborescent. The canals are 

 occupied by coecal processes of the outer mantle-layer,* and are covered ex- 

 ternally by a thickening of the epidermis. Mr. Huxley has suggested that 

 these coeca are analogous to the vascular processes by which in many asci- 



dians the tunic adheres to the test ; the extent of which adhesion varies in ■ 



closely allied genera. The large tubular spines of the Productidce must have ' 

 been also lined by prolongations of the mantle ; but their development was 

 more probably related to the maintenance of the shell in a fixed position, 



than to the internal economy of the animal. {King.) Dr. Carpenter states i 



that the shell of the BracJiiopoda generally contains less animal matter than i 



other bivalves ; but that Disci?ta and LingnJa consist almost entirely of a homy -j 



animal substance, which is laminar, and penetrated by oblique tubuli of ex- , 



treme minuteness. He has also shown that there is not in these shells that j 



distinction between the outer and inner layers, either in structure or mode of '; 



growth, which prevails among the ordinary bivalves ; the inner layers only , 



differ in the minute size of the perforations, and the whole thickness corres- i 



ponds with the outer layer only in the LameUibranchiata. The loop, or ^ 



brachial processes, are always impunctate. i 



Of all shell-fish the Brachiopoda enjoy the greatest range both of i 



climate, and depth, and time; they are found in tropical and polar sess ; in i 



pools left by the ebbing tide, and at the greatest depths hitherto explored by ' 

 the dredge. At present only 70 recent species are known; but many more" ^ 

 will probably be found in the deep-sea, which these shells mostly inhabit. 



The number of living species is already greater than has been discovered in j 



any secondary stratum, but the vast abundance of fossil specimens has made \ 



them seem more important than the living types, which are still rare in the " 



cabinets of collectors, though far from being so in the sea. Above 1,000 ,i 



extinct species of Brachiopoda have been described, of which more than half < 



are found in England. They are distributed throughout all the sedimentary / 

 rocks of marine origin from the Cambrian strata upwards, and appear to have 



attained their maximum, both of generic and specific development, in the - 

 Devonian age.* The oldest form of organic life at present kno\vn, both in 



the old and new world, is a Lingula. Some species (like Atrypa reticularis) j 



* Called the "lining membrane of the shell," by Dr. Carpenter. (Davidson Intr. ^ 



Mon. Brach.) INIr. Quekett states that the perforations are closed externally by disks, \ 



surrounded by radiating lines, supposed to indicate the existence of vibratile cilia in | 



the living specimens. \ 



\ The number of Devonian species amounts to 300 ; but these were not all j 



living at one fiwe, they are obtained from a whole series of deposits, representing a ^ 



.succession of periods. ' 



