94 



Biology in America 



have developed structures known as trichcysts. or secretions 

 contained in the surface protoplasm, which when ejected form 

 a mass of tangled threads and serve as an abattis to repel the 

 attacker. 



Among the unicellular plants also high degrees of special- 

 ization occur, which are represented mainly by variations in 

 general body form, by development of shells, filaments and 

 spines, and by changes in chromatophores and nuclei. Typ- 



AMCEBA PROTEUS, ONE OF THE MOST PRIMITIVE TYPES OF LIFE 



Photograph of a model in the American Museum of Natural History 

 in New York. 



Courtesy of the Museum. 



ically spherical or ovate in form the algae may become linear, 

 club-shaped, discoid, spiral or crescentic, while the shell mark- 

 ings of desmids and diatoms are among the most delicate and 

 beautiful objects of microscopical study. 



The chromatophores or chlorophyl carriers of the algas are 

 one of their most specialized features. In its simplest form 

 the chromatophore is disk- or plate-like, but it varies from the 

 more generalized form to the specialized star-shaped, spiral 

 or netted ones. Typically possessing but a single nucleus, 



