76 



BRACHIOPODA. 



Fig. 38. — Pelagic Brachiopodan larva 

 of Desterro, swimming by means of 

 the extended lophophore (after F. 

 Mdller). a, eye-spots; b, pro- 

 visional setae ; e, epistome ; o, oto- 

 cyst ; p, internal skeletal plate. 



of the shell. In Lingula, for instance, the peduncle projects as a 

 prolongation of the longitudinal axis of the body between the 

 posterior ends of the shell-valves, which consequently are of some- 

 what the same shape. In most other 

 forms, the peduncle is inserted more 

 in the region of the ventral valve, 

 the consequence being that, as it 

 shortens, the two valves come into 

 close but dissimilar relation to the 

 substratum. Here we may perhaps 

 find the reason for the valves having 

 become more and more heteromor- 

 phous. 



As a rule, forms in which the 

 peduncle is long and the body com- 

 paratively movable, have long and 

 posteriorly pointed shells with a short 

 hinge-line {Terebratulina). A shorten- 

 ing of the peduncle brings the posterior 

 end of the body into contact with the 

 substratum, and this leads to the 

 development of broad forms with lengthened hinge-line (e.g., Argiope, 

 Terebratella). When, as in the Discinidae, the position of the body 

 is strictly horizontal, the ventral valve resting on the substratum, and 

 the peduncle emerging from an aperture near its centre, the shell 

 resembles a circular disc (Fig. 

 39 B). Here, as in Anomia, 

 among the Lamellibranchiata, 

 there is a tendency to the 

 development of a radial type 

 of shell as a consequence 

 of the attached manner of 

 life. In the Oyster-like forms 

 that fuse with the substratum 

 (Thecidium, Crania), the diver- 

 gence in the shape of the two 

 valves reaches its highest limit. In one member of the Product id nc 

 (Probo8cidella), the ventral shell-valve becomes lengthened by the 

 growth of its frontal and lateral edges in such a way as to form a 

 calcareous tube resembling that of Aspergillum. This form of shell 

 may perhaps have arisen in adaptation to a boring or digging habit. 



A ,/> 



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Fio. 39.^Two ontogenetic stages of the shell of 

 Orbiculoidea minuta (after Beecher). A, Pater- 

 ina-stage. B, older stage, p, protegulum. 



