DEFINITIONS OF TYPES. 



PROTOZOA. 



Unicellular animal organisms. Reproduction by fission and gemmation, 

 also by conjugation, which may simulate sex-differentiation. Colonial forms 

 may show polymorphism (with division of labor) and thus serve as transition- 

 forms between Protozoa and Metazoa. 



B. METAZOA. 



Multicellular animal organisms wnth sex-differentiation. The individual 

 begins as a single cell, formed by the union of two half cells (egg and sper- 

 matozoon) produced by the parents. 



During development all metazoa pass through a blastula and gastmda stage, 

 supposed repetitions of ancient blastaea and gastraea forms. 



DIVISION I. PROTAXONIA {Coelenterata). 



"Gastraea animals." i. e. forms with persisting gastraea characteristics, 

 the primary axis, the i)rotostoma and the gastrocoele. from which all the cav- 

 ities of the body may be derived. No coelom. 



Type I. Porifera. 



G-astraea animals, sessile by the protostoma. Secondary excurrent opening, 

 osculiim. at the apical pole : with niimerous lateral pores in the body wall. 

 Middle laj'er (mesenchjana) present, dei-ived from the primary endoderm. In 

 this are produced spicules, and the germ cells. 



Type II. Cnidaria. 



Gastraea-animals. mostly marine, free-swimming or sessile at the aboral 

 pole. Develop tentacles around the protostoma. Certain specialized ecto- 

 derm cells form Cnidoblasts (nettle cells). These are most numerous on the 

 tentacles and serve as weapons. Middle layer a supporting lamella, seldom 

 cellular. Germ cells from ectoderm or endoderm. 



