THE MUSCLES OF BRACHIOPOPA. 



261 



f... 



Fig. 5. A, VENTRAL ; B, DORSAL VALVE OF 



SHOWING SCARS OF MUSCULAR ATTACHMENTS. 



(After David**.) 



a', apex ; /, foramen ; p, septum ; t, teeth ; s, sockets ; /, loop; a, adductor; 

 cc, divaricator ; 6, ventral adjuster : V, peduncular muscles. 



process, the loop (I), which forms the internal skeleton and support of the more or less developed 

 brachial or breathing organs, "the arms." This delicate calcareous appendage formed of the same 

 substance as the outside shell in reality consists of slender prolongations of the shell lip. 

 It varies considerably in size, shape, and method of attachment to the interior of the dorsal 

 valve, and is specially characteristic of the inferior or Clistenterate group. The valves are firmly 

 attached to each other by two curved hinge-teeth (A, t), which fit into corresponding depressions or 

 sockets in the opposite dorsal valve (B, s) in the articulated species. When thus secured the united 

 valves cannot be easily detached, but they are opened readily by the action of the shell-muscles, five 

 pairs of which are developed for that purpose (Hancock). Two pairs of adductors, a, close the 



valves. Another pair, divaricators, open 

 them, c, c, and two others, the ventral ad- 

 justors b, and the peduncular b', adjust the 

 shell upon the peduncle. 



Among the higher or Tretenterate group 

 the muscular system is very complicated,, 

 and in the Discinida? (Fig. 2) approaches 

 nearest to that of the articulated genera. 

 In the helmet shells (Craniadse, Fig. 9)< 

 the valves move upon the straight side, as 

 on a hinge without sliding, but in Lingula, 

 (Fig. 7) they have been observed to slide 

 AUSTRALTS, from side to side. This genus has five pairs 

 of muscles, and an odd one, their functions 

 being thus apportioned : The single muscle 

 in the region of the beak, the umbonal 



(Fig. 6, g) opens the valves. The pair of centrals (K) closes them ; three pairs of laterals (j, k, I) 

 are restricted to the side of the valves, and slide them ; while the fifth pair of transmedians (t) 

 controls the movements from side to side of the beak or umbonal regions of the shell. (King.) There 

 are no peduncular muscles. The muscles leave definite arched-, crescent-, or otherwise-shaped: 

 impressions on the interior of the valves at the point of their attachment to the shell. These scars, 

 recognisable even in many fossil forms, serve as additional 

 clues in referring a genus to one or other of the particular 

 family groups. 



The peduncle of the Tretenterates passes either through 

 a fissure in the ventral valve, as in Discinisca (Fig. 2), or >'" 

 between the beaks of the almost equivalved shell, as in 

 Lingula (Fig. 7), in which this mooring organ is sometimes 

 enormously developed, occasionally measuring over nine 

 inches. It is mobile, ringed,* highly contractile, and com- 

 posed of an outer horny layer, and an inner one of longitu- 

 dinal muscular fibres. It is hollow, and the blood, which 

 is of a red colour, courses back and forth in the central 

 cavity, circulating for several days pfter death and separation 

 from the thoracic portion of the animal. Its surface is 

 crowded with minute pores, and in two species (Lingula 

 pyramidata f and L. anatina), which live free in the sand, and do not adhere by its extremity, has the 

 power of agglutinating a sand tube. This is promptly repaired when broken or removed from the 

 animal, an operation thus described by Professor E. S. Morse : " When the peduncle was broken off, 

 a bulb of sand would soon be agglutinated to protect the broken end, and not only sand was used, but 

 bits of seaweed ; and in one case a little stick was incorporated in this structure. I brought home 

 with me to Salem, Mass., a number of living specimens, and these were kept alive in large bowls, from 



* According to researches of W. K. Brooks ("Results of the Chesapeake Laboratory, 1879"), the segmentation of the 

 peduncle in Lingula described by Morse is not a permanent character. f Now known as Glottidia audelarti, Broderip. 



Fig. 6. A, VENTRAL J B, DORSAL VALVE CF 



LINGULA ANATINA. (After King. ) 



g, Umbonal Muscles ; ft, Centrals ; }, k, I, Laterals ; 

 j, Transmedians. 



