6l6 THE INVERTEBRATA 



directed between the tentacles of the lophophore, and the smaller and 

 lighter particles suspended in it are sieved away and pass into the 

 ciliated buccal groove and so towards the mouth. Heavier particles 

 drop on to the ventral mantle lobe and are removed by outgoing ciliary 

 currents and sudden clapping movements of the valves. When the 

 ingoing currents have passed between the spirals of the lophophore 

 they unite in the median dorsal part of the mantle cavity and become 

 the outgoing current. The lophophore of Testicardines is supported 

 by calcareous processes of the dorsal valve (the brachial skeleton) 

 which assumes diverse and diagnostic forms in the different genera. 



The mouth leads into a ciliated alimentary canal. There is a stomach 

 into which opens the digestive gland composed of branching tubes in 

 the cavity of which most of the digestion takes place. In Waldheimia 

 the intestine ends blindly, but in Lingula and Crania there is an anus. 

 The coelom is spacious and divided into a right and left half by a 

 dorsoventral mesentery; transverse mesenteries also exist. It is pro- 

 longed into the lophophore and tentacles and into the mantle as the 

 pallial sinus. A pair of segmental organs, short tubes with large 

 nephrostomes, which also function as generative ducts, are situated 

 in the coelom; their external openings are at the sides of the mouth. 

 The generative organs are developments of the coelomic epithelium 

 and eggs and sperm alike dehisce into the body cavity. The sexes are 

 usually separate in the brachiopods. 



The blood system is very little developed and consists only of a 

 longitudinal vessel in the dorsal mesentery, in one region of which a 

 contractile vesicle may be distinguished as the heart, and a number 

 of vessels running forward to the mouth and backward to the mantle 

 and generative organs ; all end blindly. 



The nervous system mainly consists of a supraoesophageal and 

 a suboesophageal ganglion in front of and behind the mouth respec- 

 tively, connected by circumoesophageal connectives. A nerve runs 

 to each tentacle but no special sense organs are known. 



Lingula (Figs. 426, 427 H) is a persistent form, which has lived since 

 the earliest period of which we have an organic record, the Cambrian, 

 in precisely the same stage of development, if we can judge from the 

 hard parts. It lives in mud or sand and has a very long contractile 

 stalk by which it roots itself and can withdraw from the surface. The 

 opening of the shell is usually situated near the surface and the mantle 

 secretes chaetae, like those of annelids, which project from the an- 

 terior border, and with the help of mucus and the mantle border form 

 inhalant siphons at the side and an exhalant siphon in the middle. The 

 shell valves are equal in size and horny in consistency, being composed 

 of alternating layers of chitin and calcium phosphate. 



Crania (Fig. 425) is a form without a stalk. The ventral valve is fiat 



