86 INVERTEBRATE ZOOLOGY 



The different individuals of a colony have arisen by a process 

 of budding from the individuals below them in the colony. 

 The oldest individuals are thus those nearest the base of the 

 colony, the basal one being the progenitor of the entire colony. 

 This is also the only individual which has not come into exist- 

 ence by a process of budding ; it began its life as a free- 

 swimming larva which was hatched from an egg. 



The zooecium. Mount a small portion of the colony containing 

 two or three branches on a slide under a cover-glass. 



Exercise l. Draw a large and accurate outline of the zooecia, 

 leaving out the polypides. Observe very carefully the 

 boundaries of the zooecia and their relations to one 

 another. 



The polypide. Study a number of polypides, both retracted 

 and extended. The forward end of the polypide consists of a 

 circular ridge, called the lophophore, which bears a row of long 

 ciliated tentacles. In the midst of the circle is the mouth. The 

 tentacles are very vibratile and serve as respiratory as well as 

 prehensile organs. It will be seen that the lophophore can be 

 entirely withdrawn within the zooecium. 



The digestive system. The mouth opens into the pharynx, 

 which leads into the oesophagus. This opens into a large sac- 

 like stomach, the lower portion of which is lengthened into a 

 long pouch. From the upper end of the stomach, near the base 

 of the oesophagus, the short intestine leaves it and passes to the 

 thick-walled rectum. This leads to the anus, which is situated 

 just outside the lophophore near the mouth. The digestive tract 

 has thus the shape of the letter V, the point of which is formed 

 by the stomach pouch. Passing from the stomach pouch to the 

 lower end of the body is a broad mesenteric strand called the 

 funiculus. In order to study the digestive tract satisfactorily, 

 a polypide should be found in which both arms of the V come 

 into view. 



