CELL AGGREGATION AND DIVISION OF LABOR 



101 



rounded structures which serve as receptacles for the fertiUzed eggs. In 

 Caberea (Fig. 66, B), another bryozoon, are individuals of a fourth 

 type, the vibracularia, which serve as tactile threads. The avicularia, 

 vibracularia, and ovicells are individuals which unlike the nutritive 

 zooids consist of, the body wall alone, the intestine and tentacles having 

 been lost. 



Although among the Annelida (segmented worms) colon}' formation 

 is rare, one species, Syllis ramosa (Fig. 67), by lateral budding produces 



Fig. 67. — SylHs ramosa, a polychsete worm, with lateral buds. Two young bucLs are 

 shown at top, right. Only a small portion of the colony is shown. (After Mcintosh 

 in Challenger Reports.) 



a branched colony from which sexual zooids, male and female, finally 

 arise. The sexual zooids later separate from the parent colony. 



The Cestoda of which any tapeworm (Fig. 68) is a good example 

 form colonies by fission in a budding zone near the scolex or so-called 

 head. Each segment, or proglottis, is to be regarded as an individual. 



The Tunicata (a group of degenerate animals related to the verte- 

 brates) give rise to colonies of various sorts. The colony may be a more 

 or less regular encrusting mass as in Botrjdlus (Fig. 69), or it mixy be in 

 the form of a hollow cylinder as in Pyrosoma (Fig. 70). The zooids arc 

 embedded in a common gelatinous mass and each zooid may or may not 



