196 



ZOOLOGY. 



viduals of the colony are held together. Each member of the colony 

 may retire into its own particular portion of the exoskeleton, when dis- 

 turbed, by the contraction of appropriate muscles. The brain consists 

 of a single ganglion lying between the mouth and anus. The two sexes 

 usually occur in the same individual. The colonies are formed by budding, 

 which takes place in each species in a way that is characteristic of that 

 species. Thus it comes about that the colonies differ as much in general 

 appearance as their individuals do in structure. 



FIG. 94. 



FIG. 94. A fresh-water polyzoan, Plumatella. From Parker and Haswell, after 

 Allman. a, anus; fu., funiculus, a band of tissue anchoring the intestine to the body 

 wall; g, ganglion; int., intestine; m, mouth; o, oesophagus; r, reproductive gland; rt, 

 retractor muscle; st, stomach; stat, statoblast; t, tentacles. 



Questions on the figure. Is this an individual or a colony? What is 

 the function of the retractor muscles? To what degree are the polyps 

 capable of contraction as shown in the figure? The value of this power? 

 What are the statoblasts? 



Class 2. Brachiopoda (arm-footed; lamp-shells}. Brachiopods are 

 marine forms chiefly of geological interest, as there are at present only a 

 few living species. They were very prevalent in early geological times. 

 They possess a bivalved shell which suggests that of the bivalve Mollusca 

 (as the clam). From this external resemblance they have long been 

 classed as mollusks. The valves are strictly dorsal and ventral in the 

 Brachiopods, however; whereas in the mollusks they are right and left. 

 Their shell is therefore no longer considered as homologous with the 

 mollusk shell. The internal structure is still further removed from that 

 of the clam. The digestive tract is often bent much as in the Polyzoa, 

 ancl the mouth is surrounded by a tentacle-bearing lophophore (the 



