484 The Animal Kingdom 



The moss animals represent the only surviving members of a very 

 ancient group dating from Cambrian times. 



The Phylum Brachiopoda. — The members of this phylum, like 

 those of the Bryozoa, have an ancient lineage which traces back to 

 Cambrian times. Lingula (Fig. 159,D), a common genus, is the oldest 

 of all living genera of animals. All are marine forms with most living 

 in shallow waters. 



They are characterized by their having a bivalve shell which, how- 

 ever, opens differently from that of the bivalve molluscs. One valve 

 represents the ventral half of the animal, the other the dorsal. They 

 are hinged at the posterior end. Most species have a short stalk by 

 which they attach themselves to a substrate. They are commonly called 

 the "lamp shell" due to their resemblance to old Roman lamps. 



Inside the shell which is secreted by a mantle there is a coiled 

 lophophore, a well-developed coelom, a small heart with a few blood 

 vessels, two nietanephridia for excretion, a nerve ring, and either a com- 

 plete or incomplete digestive tract. 



The brachiopods feed on smaller organisms which are brought 

 into the shell by currents set up by the cilia on the tentacles. Mucus 

 entangles the material which is then swallowed. 



Sexes may or may not be separate, but in most, the developing 

 embryos remain within brood pouches for a time. After escaping, chey 

 swim about for a brief period of time before changing into the adult ani- 

 mal. The larvae have eyespots which disappear when they become 

 adults. 



The relationships of the brachiopods are very difficult to interpret. 

 Their trochophore larva relates them to the annelids and molluscs ; how- 

 ever, in some species, the coelomic pouches arise as outpocketings from 

 the archenteron. This perhaps relates them to the echinoderm-chordate 

 stock. Some species, however, do develop the coelom as a schizocoel. 

 Further relationships to the annelid-molluscan stock are indicated by 

 the fact that the mouth arises from the region of the blastopore. 



The Phylum Phoronida- — The members of this phylum (Fig. 

 159,^) are small, wormlike forms which live in tubes in the shallow 

 waters of the ocean. The individual is housed in a membranous tube 

 which is in contact with the tubes of numerous other individuals. Thus 

 they often form dense colonies. 



