6 BRACHIOPODA. 



not been satisfactorily made out. Mr. Hancock sup- 

 poses that they maintain the vitality of the 'shell, and 

 that perhaps by their means any injury to it may be re- 

 paired. Some provision of this sort appears to be neces- 

 sary, because the Brachiopods do not, to any great ex- 

 tent, thicken their shells by successive internal layers, 

 like bivalve Mollusca. The shells of the Brachiopocla 

 are never provided with an epidermis ; and this may also 

 account for their perforated structure. Many bivalve 

 shells, such as those of Astarte, from which the epider- 

 mis has been accidentally removed, peel off, or become 

 eroded near the beaks to such an extent that, if new 

 layers were not continually being secreted from within, 

 the animals would be laid bare and exposed to untold 

 dangers. Sponges, Balani, Serpulce, and other extra- 

 neous organisms are often seen covering or attached to 

 the shells of Terebratulce. The hinge in the articulated 

 kinds is so firmly interlocked, that it is impossible to 

 separate the valves without using a slight degree of force. 

 They appear to be in some measure sensible of light. 

 Lacaze-Duthiers gives an instance of Thecidia collapsing 

 suddenly when his shadow passed between them and the 

 sun. He succeeded in keeping these curious Brachio- 

 pods, in a lively and active state, for six weeks by merely 

 changing the water every day. The sexual nature of the 

 Brachiopoda is not quite determined. Dr. Gratiolet sug- 

 gests that the same individuals may become successively 

 male or female at different periods. Mr. Hancock con- 

 siders Lingula, at least, to be androgynous or monoecious ; 

 and he infers from analogy that both sexes are combined 

 also in the articulated Brachiopods. But Lacaze-Duthiers 

 has investigated this part of the subject more recently 

 and under circumstances more favourable than seem to 

 have fallen to the lot of any other writer. His elabo- 



