36 CCELENTERATA. 



mouth appendages. Gonads are not developed at the time of 

 liberation. Study the swimming movements. 



Make drawings to illustrate development and adult structure 

 of medusw. 



HYDRACTINIA. 



This form is particularly abundant at Woods Hole on the 

 shells of gastropods inhabited by hermit crabs, but at certain 

 seasons is abundant on rocks or pebbles and sometimes on piles. 



1. Examine a shell covered with a colony, and notice the 

 distribution and size of the individuals. 



2. Notice the hard secretion that sticks up as prominent 

 points and ridges between the individuals. 



3. Break a shell and place the fragments incrusted side up 

 in a watch-glass of sea-water and examine with a low power. 

 Three kinds of individuals will be apparent : (a) Large individuals 

 with long tentacles. These are the feeding hydranths. They 

 differ somewhat in appearance in the male and female colony. 

 The male individual has a large proboscis, while the female in- 

 dividual has only a slightly arched disc with the mouth in the 

 center, (h) Reproductive individuals with knob-like tenta- 

 cles, a proboscis that is usually retracted, a mouth, and with 

 gonophores along its sides. In female gonophores the manu- 

 brium and a number of eggs may be seen. These gonophores 

 never become detached and never show further medusoid 

 structure, (c) Elongated individuals, especially near the out- 

 skirts of the colony, that have rounded tentacles, proboscis, 

 and mouth like those of the reproductive individuals. These 

 sometimes branch and have a habit of bending the head toward 

 the base or even twisting the body into a spiral. They are not 

 distinguishable from the reproductive individual except by 

 shape and the fact they have no gonophores. 



4. Notice that the individuals are connected at the bases 

 by a fleshy layer which is responsible for the deposit already 

 mentioned. 



Make a drawing of each kind of individual. 



