26 Laboratory Guide in Zoology 



ing, and the free-swimming jelly-fish form. For this 

 reason the species is said to have an alternation of 

 generations. 



XI. — In what part of the gonangium are the medusa-buds 

 largest .■' 



XII. — Are there any tentacles in connection with the 

 gonangium, or is there any way in which it can get food 

 from outside ? If not, where does the substance of 

 which the medusa-buds are built up come from ? 



MEDUSOID STAGE OF A CAMPANULARIAN HYDROID 



I. — In your study of the hydriform stage of the Campanu- 



larian Hydroid you observed, in the gonangia, small 

 bodies called medusa-buds. If you saw well-developed 

 medusa-buds, you found them to be bell-shaped. When 

 mature, these buds break away and escape through the 

 opening in the distal end of the gonangium into the 

 water, where they become at once independent, frpe- 

 swimming animals. 



II. — Compare this free-swimming Medusa to an umbrella 

 with the handle broken off short. The short handle is 

 the manubrium. In its end there is an opening, which 

 is the mouth. From the base of the manubrium to the 

 margin of the circle run four ribs of the umbrella : 

 these are radial canals. They open into a circular canal. 

 Stretching horizontally from the circular canal toward 

 the middle is a thin white membrane : this is the velum. 

 The convex side of the umbrella is called the exumbrella 

 or aboral surface, and the concave side is the subumbrejla 

 or oral surface. 



Ill- — Projecting downward from the outer margin of the 

 umbrella are many rather long threads : these are the. 

 tentacles. About how many of them are there ? 



