374 BBACHIOPODA [CH. 



organs, such as ears or eyes, are known ; and indeed the fixed 

 Brachiopod, whose "strength is to sit still" and sweep little particles 

 of food towards its mouth, has but little need of specialised sense- 

 organs. 



The cavity of the coelom is reduced by the presence of the 

 alimentary canal, the digestive gland and the heart, but it is still 

 spacious. It is partially divided up by certain mesenteries which 

 support the alimentary canal, and it is traversed by several pairs of 

 muscles. Some of these muscles run from valve to valve, and when 

 they contract close the shell, others being situated behind the hinge, 

 so that when they contract the valves slightly open. Others again 

 run from the valves to the inner surface of the stalk and their con- 

 traction bends the body one way or another and may even serve to 

 slightly rotate it. There are two large canals in the lophophore, one 

 on each side which are also almost certainly coelomic in character, 

 although in the adult they are shut off from the main coelom. 



There is except in one genus but one pair of excretory organs. 

 These are short tubes which open by large trumpet-shaped orifices 

 (9, Fig. 175) into the coelom ; while their external openings are 

 situated at the sides of the body behind the lophophore. The cells 

 lining the tubes are some of them ciliated, while others are crowded 

 with coloured granules. As already mentioned, the genus Rhyn- 

 chomlla possesses two pairs of such organs. 



As a rule, in Brachiopods the sexes are separated. The cells 

 destined to form ova or spermatozoa are derived from those lining 

 the body-cavity. At certain places, usually four in number, the 

 coelomic cells multiply and build themselves up into ovaries or testes 

 according to the sex. When they are ripe they fall off into the 

 coelom and make their way to the exterior through the excretory 

 organs which thus also serve as genital ducts. The spermatozoa 

 are cast into the water by the male, and the female must bring 

 them within the valves of her shell by the action of the current set 

 up by the cilia, because the eggs are almost certainly fertilised as 

 soon as they leave the excretory organs. The eggs develop in 

 certain brood-pouches situated at the sides of the animal which 

 are formed by a bulging in of the body-wall. A larva is ultimately 

 formed which leaves the body of the mother and swims about in 

 the sea by means of a band of cilia. As it is extremely minute, 

 although it swims q-uickly it does not get very far, and this probably 

 accounts for the fact that Brachiopods are usually found in large 

 numbers in one place. 



