100 Biology in America 



side of an ordinary fern leaf he will find its edges pimpled 

 with rows of little brown capsules somewhat smaller than a 

 pin head, which on bursting scatter to the wind a fine brown 

 dust. This consists of the spores, which after germination 

 produce a leaf-like body, the prothallus, about a quarter of 

 an inch in diameter. This is the gametophyte, which bears 

 the sexual organs. It grows only in moist places, moisture 

 being necessary for the transfer of the sperm to the egg. 

 From the fertilized egg develops the sporophyte or ordinary 

 fern plant, thus completing the cycle in the life of the fern. 



Alternation of generations also occurs in some algae. Here 

 it is the gametophyte which is the conspicuous plant, the 

 sporophyte being usually a smaller structure. 



Passing upward from the lower to the higher plants we 

 see then the sporophyte progressively increasing and the 

 gametophyte decreasing in importance. 



While alternation of generations is characteristic of plants 

 it occurs occasionally among the many-celled, as well as in 

 unicellular animals. Many of the delicate and beautiful jelly- 

 fish, with which any observant visitor to the seashore is 

 familiar, are the sexual phase of the life cycle of an animal 

 whose asexual form consists of an attached series of disks, 

 which in the course of development separate from one another 

 to form the sexual form or medusa. In certain marine worms 

 (Polychaeta) also alternation of generations occurs. The 

 anterior part of the body does not develop sex organs, while 

 posteriorly the worm divides into several part ;, wliieh becom- 

 ing sexually mature separate from the parent stock to form 

 the sexual generation. In some of the curious "sea squirts" 

 or tunicates also this process is found. 



The tunicates derive their name from the mantle or tunic 

 surrounding the body. Some are fixed, and others free swim- 

 ming as adults ; while in the former the animal is frequently 

 free-swimming as a larva. The name of "sea squirt" is 

 derived from the habit of the fixed forms of squirting out a 

 stream of sea water when touched. 



The larva of the fixed forms is totally different from the 

 adult and the true relationships of the latter could not be 

 understood were it not for the existence of the former. This 

 is a tadpole-like animal with a long tail through which runs 

 a supporting rod, the notochord. At the anterior end of the 

 animal is an adhesive disk by means of which it attaches 

 itself at the time of metamorphosis. The wall of the pharynx 

 is perforated by a number of openings or gill slits which lead 

 into a waste chamber or atrium opening to the exterior by 

 a pore. Dorsal to the pharynx is a nervous mass or primitive 

 brain, and between the two a small duct opening into the 



