CLASS II 



BRACHIOPODA 



301 



vertical plane as on hinges, and at the same time prevents motion in a lateral 

 direction. The diductors, being attached to the cardinal process, act upon a 

 ,-irm when they contract, thus opening the valves, 

 while the contraction of the adductors serves to close 

 them (Fig. 497). 



In the Atremata and Neotremata the muscles are 

 arranged differently, arid are often more complicated 

 and numerous, as articulation is usually not present 

 in these orders. The greatest complexity is attained 

 in Lingula (Fig. 499), 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 growth from the 

 egg to that condition in which the animal is 

 recognisable as possessing some distinctive class 

 characters ; and (b) from the first shelled condi- 

 tion, or protegulum, to maturity and old age. 



Our knowledge of the earliest embryonic condi- 

 tions is restricted to Terebratulina, Liothyrina, Cistella, 

 and Thecidea. After fertilisation the larvae may remain 

 attached, and pass 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 growth before the larvae can be definitely recognised as Brachiopods. 

 These are : (1) The Protembryo, which includes the ovum, its segmented stages preced- 

 ing the formation of a blastula, or primary 

 internal cavity (Fig. 500, A, B) ; (2) the Mes- 

 embryo, or blastosphere, a multi-segmented larva 

 with an internal cavity (Fig. 500, C) ; (3) the 

 Metembryo, or gastrula stage (Fig. 500, D) ; 

 (4) the Neoembryo, or segmented ciliated 



ABC D 



FIG. 501. 



Cistella Neapolitana, Schacchi. A, Neoembryo ; 

 embryo of two segments. 11, Neoembryo ; cepha- 

 lula, 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 (<0 and ventral (<) 

 mantle lobes, and umbrella-like cephalic segment. 

 (A and Ji after Kovalevski, C after Shipley ; all 

 reproduced from drawings by Beecher.) 



FIG. 499. 



Lingula anatina, Brug. Recent. 



A, Shell with pedicle, natural size 



B, Interior of ventral valve show- 

 ing muscular impressions ; a, Ad- 

 ductors ; c, Protractors ; p, Re- 

 tractors ; o, Pedicle muscle. 



FIG. 500. 



Cistella Neupolitunn, Schacchi. Recent. A, 

 Protembryo ; unsegmented ovum. B, Protembryo ; 

 ovum composed of two spheres. C, MeseniKrv \ ; 

 blastosphere. D, Metembryo ; Gastrula v.it'tcr 

 Shipley, from Beecher). 



Cephalula, which consists at first of a cephalic lobe, bearing eyes in Cistella, and a 

 caudal lobe, to which is added later a thoracic segment carrying four bundles of 

 setae. At the same time the dorsal and ventral sides of the latter segment become 

 extended over the caudal lobe, and are progressively defined as two lobes (Fig. 501) ; 

 (5) the Typembryo, or larval stage, in which the dorsal and ventral thoracic lobes, 

 or mantle, fold over and enclose the cephalic lobe (Fig. 502, B). Upon the mantle 

 lobes, either before or after turning, there is a corneous integument which develops 

 into the protegulum. before the formation of the true shell. The caudal segment 



