34 CCELENTERATA. 



and a somewhat enlarged ccelenteron. (b) A bud showing a 

 thickening of the ectoderm at the distal end, in which a cavity 

 appears, the sub-umbrellar cavity, (c) A bud showing the 

 formation of the manubrium as a projection into this cavity. 

 The manubrium involves both layers, as the sub-umbrellar 

 cavity is wholly ectodermal. The ectodermal distal covering 

 of the sub-umbrellar cavity will later perforate and form the 

 velum, (d) A bud showing the perforated velum and the 

 tentacles. The tentacles are at first directed through the open- 

 ing of the velum into the sub-umbrellar cavity. 



4. Find medusce that have become detached. Notice the 

 arrangement and number of tentacles, the eye spots at the 

 bases of the tentacles, the radial and circular canals, and the 

 mouth appendages. Gonads are not developed at the time of 

 liberation. Study the swimming movements. 



Make drawings to illustrate development and adult structure 

 of medusce. 



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. (6) 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 



