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 lever 
arm 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, and 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- Fra. 499. 
tions is restricted to Terebratulina, Liothyrina, Cistella, Lingula anatina, Brug. Recent. 
and Thecidea. After fertilisation the larvae may remain = Se ee Ne eta gar ae 
attached, and pass their early stages within the parent ; ing muscular impressions; a, Ad- 
or they may develop cilia before segmentation, and be ee : ae veatbieni welts ete 
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 i 
internal cavity (Fig. 500, A, 6) ; (2) the Mes- 
embryo, or blastosphere, a multi-segmented larva (2) 
with an internal cavity (Fig. 500, C) ; (3) the ifs 
Metembryo, or gastrula stage (Fig. 500, D) ; We 
(4) the Neoembryo, or segmented ciliated 

Fia. 501. 
Cistella Neapolitana, Schacchi. A, Neoembryo; 
embryo of two segments. B, Neoembryo; cepha- 

Fic. 500 lula, ventral side, showing cephalic, thoracic, and 
; caudal segments, eye-spots, and bundles of setae. 
Cistella Neapolitana, Schacchi. Recent. 4, C, Neoembryo; lateral view of completed cephalula 
Protembryo ; unsegmented ovum.. B, Protembryo ; stage, showing extent of dorsal (d) and ventral (7) 
ovum composed of two spheres. “C, Mesembryo ; mantle lobes, and umbrella-like cephalic segment. 
blastosphere. D, Metembryo; Gastrula (after (4 and & after Kovalevski, C after Shipley; all 
Shipley, from Beecher). reproduced from drawings by 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 seement 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 
