BOUGAINVILLIA, HYDRACTINIA 53 



1. Examine the colony for arrangement of branches, and 

 determine the relation of perisarc and coenosarc. 



2. How do the hydranths differ from those of Obelia? Is 

 the number of tentacles constant? Is the hydranth as con- 

 tractile as it is in Obelia? 



3. Look for gonosomes. The gonophores are borne singly 

 or in clusters on the main stem and branches. By examining 

 a number of buds the general method of medusa formation can 

 be determined. If possible, find: (a) a young bud slightly 

 swollen showing the thin perisarc with the cellular layers in- 

 side and a somewhat enlarged coelenteron. {b) A bud show- 

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

 cavity appears, the subumbrellar 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 subumbrellar 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 opening of the velum into the subumbrellar cavity. 



4. Find medusae 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 struc- 

 ture of medusae. 



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. 



