482 



ESSENTIALS OF ZOOLOGY 



Colony Formation in Certain Protozoa 



Due to incomplete separation of cells following division in Pro- 

 tozoa, colonies are begun. In some instances groups of cells in some 

 such colonies, e.g., volvox, become specialized as gametes for repro- 

 ductive purposes and the other cells remain as somatic cells. Be- 

 cause of the similarity between various mastigophoran colonies and 

 blastula stages in metazoan development, this group has frequently 

 been cited as the predecessors from which Metazoa at some time 

 originated. 



Development of the Gastrula 



The primitive gastrula is thought to have had its origin from a 

 spherical colony of Protozoa by a more rapid growth of the cells 

 at one pole which brought about an infolding or invagination to 

 produce a new cavity from the exterior. This development estab- 

 lishes a body composed of two general layers. Modern adult coe- 

 lenterates, like the hydra, and others demonstrate precisely this 

 condition today, with the outer ectoderm and the inner endoderm, 

 each composed of cells serving vital general functions. This allows 

 all cells surface exposure either directly on the exterior or to the 

 cavity. By this means larger and more complex organisms were 

 developed and have lived. 



Trochophore Larva 



In Platyhelminthes, Annelida, Molluscoidea, Trochelminthes and 

 Mollusca there occur larval forms of the trochophore type. Al- 

 though the surface ciliation and some other superficial features 

 differ somewhat, a direct relationship among these forms is trace- 



f^esode 

 band 



rma 



Head kidn 

 Qtocyst 



hesenc 



f or^an 



Stomach 



Pre oral 

 ciliated 

 rin$ 



Blastocoele 



">5 . , 

 vesicis 



Apical or^an 



Endoderm 

 Embryonic mu<^cle 



Prototroch 



f'lesoderm 



Telatroch 



Fig. 207. — A, Trochophore larva of Eupomotus (a polychaete annelid), side 

 View. (After Shearer.) 



B, Veliger larva of Patella (a marine snail), frontal section. (After Patten.) 

 (Drawn by Joanne Moore.) 



