540 THE INVERTEBRATA 



the spirals of the lophophore they unite in the median dorsal part of 

 the mantle cavity and become the outgoing current. The lophophore 

 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 hearty 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. 401 , 402 H) is a persistent form, which has lived since 

 the earliest period of which we have anorganic 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 anterior 

 border, and with the help of mucus and the mantle border form in- 

 halant 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. 400) is a form without a stalk. The ventral valve is flat 

 and fixed by its whole surface to a rock ; the dorsal valve is conical. The 

 tentacles of the lophophore are protruded from the shell margin. 



The Brachiopoda have free-swimming larvae which are usually 

 divided into three regions, an anterior like the preoral region of the 

 trochosphere, a median region in which the two lobes of the mantle 



