CLASS II 



BRACHIOPODA 



365 



Wlien a functional pedicle is present, there are found, in addition to the valvular 

 muscles, two pairs (one to eacli valve), and a single unpaired muscle ; these are 

 attached to the pedicle, and are called the pedicle muscles (Fig. 535, B,p,p'). The pair 

 in the ventral valve originates immediately outside of and posterior to the adductors 

 and diductors ; the pair in the dorsal valve is attached behind the posterior adductors ; 

 and the unpaired muscle lies at the base of the pedicle in the ventral valve. Only 

 the latter unpaired band, as a rule, leaves a perceptible scar in fossil specimens. 



The entire muscular system in the Protremata and Telotremata works with the 

 utmost precision. The cardinal process is received 

 between the teeth of the ventral valve in such a 

 manner as to allow the dorsal valve to swing freely 

 in the median vertical plane as on hinges, and at 

 the same time prevents motion in a lateral direc- 

 tion. The diductors, being attached to the cardinal 

 process, act upon a lever arm when they contract, 

 thus opening the valves, ^hile the contraction of 

 the adductors serves to close them (Fig. 534). 



In the Atremata and Neotremata the miiscles 

 are arranged differently, and are often more compli- 

 cated and numerous, as articulation is almost never 

 j)resent in these orders. The gieatest comj^lexity is 

 attained in Lingula (Fig. 536), because these animals, 

 in addition to the absence of articulation, slide their 

 valves laterally. ' 



Ontogeny. — The development of Brachio- 

 pods from the egg to maturity may be divided 

 into two periods : (a) stages of development from 

 the egg to that condition in which the animal is ^"t^r'^Z.!'^nr^'^: il 



recognisable as possessing some distinctive class interior of ventral vaUe showing mus- 



» 1 /7\ P ,1 r. . 1 11 1 ''^'''"'' mipressions ; n, Adductors; c, 



characters; and (6) from the nrst shelled con- Protractors; j), Retractors; o, Pedicle 



dition, or protegulum, to maturity and old age. 



Fig. 536. 



muscle. 



Our knowledge of the earliest embryonic conditions is restricted to Terebratulina, 

 Liothyrina, Argyrotheca and Thecidea. After fertilisfition the larvae may remain 

 attached, and j^ass their early stages within the parent ; or they may develop cilia 

 before segmentation, and be set free in the pallial chamber or in the sea-water. The 

 free larvae swim by the aid of cilia with a twirling motion. There are five well- 

 marked stages of develojiment before the 

 larvae can be definitely recognised as 

 Brachiojjods. These are : (1) The prot- 

 evibryo, which includes the ovum and its 

 segmented stages preceding the formation 

 of a blastula, or primary internal cavity 



B 



D 



Fi<}. 537. 



Argyrotheca neapoliiana (Schacchi). Recent. 

 A, Protembryo ; unsegmented ovum. B, Prot- 

 embryo ; ovum composed of two spheres. (.-', 

 llesembryo; blastosphere. D, MetemVjryo ; gast- 

 rula (after Sliipley, from Beecher). 



Fig. 538. 



Argyrotheca neapoliiana (Schacchi). A, Neo- 

 embryo; embryo of two segments. i>, Neoembryo ; 

 cephalula, ventral side, showing cephalic, thoracic 

 and caudal segments, eye-spots, and bundles of 

 setae. C, Neoembryo ; lateral view of completed 

 cephalula stage, showing extent of dorsal (d) and 

 ventral (v) mantle lobes, and umbrella - like 

 cephalic segment. (A and B after Kovalevski, C 

 after Shipley ; all reproduced from drawings by 

 Beecher.) 



