rHVLlM (:(ELENTEliAT.\ 11 



nucleoli rcsciiiblc tliose of the epitlielio-iiiuscuhir cells. At 

 their free ends are two flagelhi which arc not visible in ordinary 

 preparations. The basal ends of these cells are provided witli 

 I'xtensions containing contractile fibers. 



Between the digestive cells another type of cell may often be 

 seen. These appear as triangidar cells with the broader end 

 exposed to the digestive cavity. They are kiiown as ghiinl 

 cells, since they secrete the enzymes used in the small amount of 

 extracellular digestion. 



The non-cellular mesoglea secreted by cells of the ecto- 

 derm and entoderm is composed of a gelatinous non-cellular 

 material. 



If possible the details of cellular structure should be reviewed 

 by a study of mascerated, teased, and stained cells of the body 

 walls. "When such preparations are successful one should be 

 able not only to recognize the cell components of the body walls 

 but also to determine the form and general relations of the 

 cells. Sensory and nerve cells are difficult to find and study in 

 transections and are therefore not included in these outlines. 



In a four-inch circle representing a diagram of a cross-section 

 of Hydra show the relative width of ectoderm, mesoglea, 

 entoderm, and gastrovascular cavity. In a small sector of this 

 diagram show in detail the structure of the various types of cells 

 composing the two layers of the body wall. 



Class Hydrozoa (Obelia). — In addition to Hydra and a few 

 other fresh-water representatives, the class Hydrozoa includes a 

 large number of marine species of diverse form and appearance. 

 These are, however, all reducible to the fundamental plan of 

 structure typified by Hydra. Great numbers of these species 

 are colonial in habit. As an example of a colonial hydroid, Obclia 

 is chosen for study. It is a common marine form found on docks, 

 submerged or floating wood, stones, seaweeds, etc. Obelia 

 commonly occurs in shallow water just below the ebb-tide lev(>l. 

 It may be found at greater depths. 



In Obelia the striking feature is the division of labor which 

 obtains among the several kinds of indiviriiials comprising the 

 colony, each of which is specially modified in connection with 

 the performance of definite functions for the colony as a whole. 

 There are three kinds of polyps {i.e., three kinds of individuals 

 or "Obelia persons"): (1) the nutritive polyp, whose service to 

 the colony is food getting; (2) the reproductive polyp, whose serv- 



