SOME OTHER TYPES OF INDIVIDUAL ORGANIZATION 



179 



single-celled animals; Monosiga (Fig. 13.2) is a more specialized type of 

 protozoan, fixed by a stalk, and having a sticky protoplasmic collar and 

 a whip that brings food particles to the collar by means of a current. 



Some of the Protozoa show an approach to the next level of organiza- 

 tion by living together in groups or colonies of cells. Aside from certain 

 mechanical advantages gained by 

 increased size and better protection 

 of the immature cells, they are 

 scarcely different from other pro- 

 tozoans. The cells still function as 

 individual units, and all are alike or 

 potentially alike. In Proterospongia 

 (Fig. 13.3), a form closely allied to 

 Monosiga, a group of cells is enclosed 

 in a gelatinous mass. Those at the 

 surface have collars and whips like 

 Monosiga; those within can move 

 about by amoeboid movements, and 

 upon reaching the surface can 

 rapidly produce similar collars and 

 whips. There is no organization of 

 the cells. 



In more than 90 per cent of 

 all known kinds of animals the 

 individual is made up of at least 

 many hundreds and often of 

 billions of cells, and these cells 

 exhibit a greater or less amount 

 of differentiation and specializa- 

 tion for particular tasks. In all but 

 the simplest of these many-celled 

 animals (Metazoa) the cells are 

 organized into tissues, into tissues 

 and organs, or into tissues, organs, 

 and systems. 



Cellular level. The sponges (Porifera) are the simplest and least 

 highly organized of the Metazoa. Essentially they consist of two layers 

 of cells arranged to form hollow tubes or chambers (Fig. 13.4); the layers 

 are separated by a thicker or thinner layer of nonliving cell products, 

 which include gelatinous materials and supporting fibers or rodlike 

 structures. In general the cells on the outside are different in form from 

 those on the inside layer; but a cell from either layer can withdraw into 

 the interior of the mass and later take up a position on the opposite face, 



Fig. 13.2. Monosiga, a stalked protozoan 

 with flagellum and sticky food-gathering 

 collar. (Courtesy American Museum of Natu- 

 ral History.) 



