78 LABORATORY STUDIES /A' ZOOLOGY 



ice is the production of medusje which arise as buds; and (3) 

 the medusoE, whose function is gamic reproduction and the 

 dissemination of the species. A colony consisting of three or 

 more different kinds of speciaHzed individuals is said to be 

 polymorphic. 



The Structure of a Colony. — Certain general features of the 

 colony must be clearly in mind before a detailed study of any 

 part is begun. Under the dissecting microscope examine a 

 prepared specimen of a single stem of an Obelia colony. Note 

 that the main stem is zigzag in form although sometimes in 

 preserved material this is not pronounced. A demonstration 

 preparation of a stem exhibiting its natural state should also be 

 examined. The attached end of the stem, in root fashion, creeps 

 along the surface to which the colony is attached. This portion 

 of a stem may be called the "foot." 



The main stem gives off alternating side branches each of which 

 terminates in an individual of the colony. At the extremities 

 of these side branches observe the nutritive polyps, characterized 

 by the presence of numerous tentacles. The polyp nearest the 

 foot is termed the founder polyp, since it founded the colony. 



Less numerous are the reproductive polyps, which are usually 

 borne in the angles where the stems of the nutritive individuals 

 arise from the main stem. They may be distinguished from 

 undeveloped feeding polyps by their position on the stem as 

 well as by their swollen appearance. 



All the soft parts of the colony are protected by a translucent 

 sheath, the perisarc, which becomes expanded and variously 

 modified in the regions of the nutritive and reproductive polyps. 



Make a six-inch diagram showing the growth habit of an 

 Obelia colony. Use schematic figures to represent the several 

 kinds of individuals. Label structures described in the preceding 

 paragraphs. 



Details of Structures. — Under the low (or, if necessary, high) 

 power of the compound microscope study in detail a portion 

 of the stem and branches containing all types of polyps. 



The living tissues of the individuals and stems are called the 

 cosnosarc, the reference being to the living flesh in contrast to the 

 perisarc which is composed of inert materials secreted by the cells 

 of the coenosarc. 



The coenosarc of the main stem will be readily recognized 

 by its granular nature and deeper staining qualities. Note that 



