BRACHIOPODA. 



487 



of the mantle to the shell is due to the penetration of these pores by 

 minute tubular processes of the external layer of the mantle *, which 

 arc covered externally by the epidermis. The free borders of the 

 mantle-lobes are usually fringed by fine bristles, setigerous in Tere- 

 hratula\, jointed in Lingida.\ 



The muscular system is very complex. Taking the ventral or 

 perforated valve, to which the peduncle is directly attached, as the 

 more fixed point, the adductorcs longi {Jig. 187, e,f) arise from a 

 187 single pyriform area at the middle of 



that valve, a little behind a line trans- 

 versely bisecting it ; the fibres soon 



fe^Bi^L^jjB/.^ /o become tendinous, converge, and 



/ ' ^•V^^k"'''*/'^^^~^^^^^^\\ gi'oup themselves into two lateral 



muscles forming a pair ; each of these 

 muscles, as it approaches the opposite 

 valve, expands and subdivides into an 

 anterior and posterior portion; glides 

 between the stomach and the crus of 

 the calcareous loop of its own side (i), 

 and is attached by the double expanded insertion into the dorsal or 

 perforated valve (d), the anterior division (/) is the " adductor longus 

 anticus," the posterior one (e), " adductor longus posticus ;" but 

 both of the so-divided adductors, by reason of their ventral confluent 

 attachment, may be regarded as constituting one quadricipital muscle. 

 The action of this complex muscle is directly to close or adduct the 

 valves, in which action it will slightly compress the hepatic lobes 

 and stomach. The adductor brevis (g) forms a symmetrical pair, 

 having their expanded disc of attachment to the ventral valve ex- 

 tended somewhat in advance of the confluent origin of the preceding 

 muscles : the fibres of each pass obliquely backwards, and converge 

 to a small round shining tendon, the tendons passing on each side 

 the intestine to be inserted close together into the cardinal process 

 of the dorsal valve (d). Their action will be to adduct the valves ; 

 but with a more oblique movement bearing upon the ventral valve 



Tfrebratula flavescens. 



* Carpenter, Report on the Microscopic Structure of Shells, Part II., Trans. 

 Brit. Association, 1847, p. 93. " In these tubes, as will hereafter be shown, 

 certain ccecal appendages of animal membrane are situated." — Quekett, " Histo- 

 logical Catalogue," vol. i. 1850, p. 270. In the same work it is shown that 

 " each perforation has a series of radiating lines or tubes on its outer margin " 

 (p. 270). The corresponding part of the membranous tubes would resemble a 

 terminal brush of vibratile cilia. 



t CCCI. pi. 2.3. fig. 13. X CCCVIII. taf. i. fig. 13. 



1 I 4 



