MOLLUSCOIDA, TROCHELMINTHES, AND CHAETOGNATHA 187 



fore composed of ectoderm, eudoderm, and mesoderm. The nervous 

 system is centered in a ganglion or mass of nerve cells located in 

 the region of the mouth and from it, nerves extend to the tentacles. 



Reproduction is accomplished either by budding or sexually. 

 Ovaries and testes make their appearance either in the funiculus 

 or in the lining of the coelom and fertilization occurs in the body 

 cavity. The early development goes on in a modified region of 

 the zooecium, called the broad-pouch or ooecium. When the embryo 

 escapes, it is a free-swimming, ciliated larva which is similar to 

 the trochophore larva found as a developmental stage of certain 

 Annelida and Mollusca. Its form resembles certain adult Rotifera. 

 This larva becomes attached and transforms into a parent indi- 

 vidual, the zooid of which will form a neAv colony by budding. 



The branching Plumatella, which is supported by a secretion of 

 calcium carbonate, and the slimy Pectinatella, whose skeleton is 

 in the form of a gelatinous mass, are the two forms most frequently 

 found in fresh Avater. These fresh water types may be developed 

 from winter eggs, enclosed in shells, or new individuals may be 

 produced as internal buds. These buds are called statoMasts. They 

 are produced in autumn and may either float on the water or sink 

 to the bottom. They withstand the rigors of winter and are stimu- 

 lated by it. So far as is known this group has little if any eco- 

 nomic value. 



Class Brachiopoda (brak i op'o da — arm and foot) is a group of 

 marine forms, the individuals of which possess bivalve shells. For 

 this reason they are sometimes confused with the clamlike molluscs. 

 The brachiopods, however, have dorsoventral valves, while the mol- 

 luscan valves are lateral. The shell is secreted by a mantle which 

 lines the valves. The tip of the beaklike valves is penetrated by a 

 foramen which serves as an opening for the peduncle. This fleshy 

 organ makes permanent attachment to some object in the water. 

 Internally, the lophophore is a conspicuous and characteristic struc- 

 ture of this type of animal. This organ is composed of two coiled 

 appendages which bear numerous ciliated tentacles. The cilia pro- 

 duce water currents in the longitudinal groove and carry food 

 particles to the mouth. 



The digestive tract is U-shaped and is composed of the mouth, 

 lophophore, gullet, stomach, and ventrally directed intestine. This 



