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 ccecal processes of the outer mantle-layer,* and are covered ex- 

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

 these co3ca 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 Productida 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 

 that the shell of the Brachiopoda generally contains less animal matter than 

 other bivalves ; but that Discina and Lingula consist almost entirely of a horny 

 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 

 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- 

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

 brachial processes, are always impunctate. 



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

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

 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 

 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 

 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 known, both in 

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



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

 Mon. Brach.) Mr. 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. 



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

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

 succession of periods. 



