444 THE WONDER OF LIFE 



in some of the parasitic forms ; but they are too di£6.cult 

 for discussion here. As a first illustration, therefore, we 

 take a multicellular animal, the freshwater sponge. In 

 some of the freshwater sponges^ — ^w^hich form the family 

 Spongillidse, aberrant in having left the sea — an interesting 

 alternation of generations has been described by W. Mar- 

 shall and others. In autumn the sponge, which grows 

 on sticks and stones in the river or lake, sufiers from the 

 cold and from a scarcity of food. It begins to die. 

 Throughout the moribund body, however, httle companies 

 of cells group themselves together and become surrounded 

 by a protective capsule of tightly -fitting, somewhat capstan- 

 hke, flinty spicules. Each gioup is called a gemmule, 

 and while the parent dies, the gemmules survive the winter. 

 In April or May they float away from the debris of the old 

 body, and develop into new sponges. Some become short- 

 lived males, others more stable females. The ova produced 

 by the latter and fertilized by spermatozoa from the former, 

 develop into a summer generation of asexual sponges, 

 which, in turn, die away in autumn, and give rise to gem- 

 mules. The formation of gemmules is an asexual mode 

 of multiphcation, and it also secures dispersal, for the gem- 

 mules can be swept about by currents without being 

 damaged, until eventually they effect lodgment in some 

 crevice and begin to develop. 



Zoophytes and Swimming Bells. — ^Many of the 

 graceful colonies of Hydroid polyps, often called Zoophytes, 

 hberate in the summer months transparent reproductive 

 buds specialized for free-swimming. These Medusoids, 

 which are in a very general way hke miniature jelly-fishes 

 or Medusae, swim in the open water by contractions and 

 expansions of their bells. They are sexual stages in the 



