THE BRACHIOPODA. 27 



of the structural characters which I have mentioned is exceed- 

 ingly important. As I have said, the intestine is not straight, 

 but is bent upon itself (Fig. 9), and the direction of flexure is 

 such that the nervous ganglion, which corresponds with those 

 called '' pedal " in Lamellibranchiata, is placed in the re-entering 

 angle between the gullet and the rectum. In order to express 

 this relation of the nervous system to the alimentary canal, the 

 flexure of the latter has been called " neural " the side of the 

 body on which the principal ganglion is placed, and towards 

 which the intestine is bent, being the " neural " side. Whatever 

 our terminology, however, the great point is to remember that 

 the structural relation which it expresses is constant throughout 

 the Polyzoa. 



IX. THE BRACHIOPODA. 



Notwithstanding that these animals differ very much in 

 external appearance from the Polyzoa, we shall find a singular 

 fundamental resemblance of internal structure between the two 

 classes. All known Polyzoa are compound animals, that is to 

 say, the product of every ovum gives rise, by gemmation, to 

 great assemblages of partially independent organisms, or zooids. 

 The Brachiopoda, on the contrary, are all simple, the product 

 of each ovum not giving rise to others by gemmation. All the 

 Brachiopoda possess a bivalve shell a shell composed of two, 

 more or less horny, or calcified, pieces, which are capable of a 

 certain range of motion on one another, and are very commonly 

 articulated together by teeth and sockets. The proper body, 

 which is small when compared with the size of the shell, has 

 its dorsal integument produced into broad membranous expan- 

 sions, which line the interior of the valves of the shell, and are 

 called the lobes of the mantle or " pallium." The aperture of 

 the mouth is situated in the middle line, between the pallial 

 lobes, and, on each side of it, is a longer or shorter prolon- 

 gation of the body, provided with ciliated tentacula. It is 

 from the presence of these " arms " that the class has received 

 its name. The tentaculate oral disk of a PlumateUa is already 

 horse-shoe shaped (Figs. 8 and 9) ; suppose each crus of the 

 horse-shoe to be pulled out to a much greater length, and 



