288 PHYLUM ANNELIDA 



lion. Sensory structures in many cases perforate the valves. 

 Above the gut lies the heart, which is connected with blood vessels. 

 Two (or more rarely four) nephridia open near the mouth, and serve 

 also as genital ducts. The posterior region of the body often forms 

 a stalk by which the shell is moored, but in many this stalk is absent, 

 and the animal is directly attached to the substratum. The sexes 

 are sometimes separate, but perhaps some are hermaphrodite. There 

 is a metamorphosis in the development, and the larva? resemble, in 

 some respects, those of Polyzoa. 



The Brachiopods date from the earliest known fossiliferous rocks, and 

 had their maximum representation in the Ordovician and Silurian. 



