So Invcrtebrata. 



in shape, and often brightly coloured. All molluscs 

 have a digestive canal, and sometimes a complex 

 arrangement of teeth. They have like- 

 wise a nervous system consisting of a 

 ring around the fore-end of the diges- 

 tive canal, on which are formed ganglia 

 over and under the tube ; besides this 

 there are often other nerve masses and 

 organs of sense. There is a heart which 

 propels the blood, but there are few 

 or no blood-vessels, the circulation 



Lamp-shell or Te- . . ... 



rebratuia, one being chiefly carried on in the inter- 



of the Brachio- - . . , , 



pods, dorsal sur- spaces of the tissues. I here is rarely 

 much of the body-cavity to be found 

 free, with the exception of a small space around the 

 heart, which is called the pericardium, and from this 

 two short tubes pass out representing the segmental 

 organs of worms. Four classes are included in this 

 sub-kingdom. 



CLASS I. Brachiopoda. Of this class compara- 

 tivey few representatives are now living, and these in 

 few places, usually at considerable depths in the sea ; 

 but at an earlier period of the world's history they 

 were very abundant. They possess shells of two 

 valves, one of which is large, placed ventrally or down- 

 wards, and having a beak pierced with a hole, through 

 which a foot-stalk projects whereby the animal is 

 anchored. The other valve is smaller and placed dor- 

 sally ; it bears on its inner surface a delicate shelly loop 

 for the attachment of the peculiar arms from which 

 the name of the class is derived. The valves are 

 joined, either by horny matter as in the duck-bill 



