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EVOLUTION OF ANIMALS 



Part V 



oughly adjusted and dependent upon the come and go of water, a sideline and 

 blind pocket in the trend of animal evolution (Fig. 33.1). 



Structure. Leucosolenia, a simple sponge, illustrates the fundamental char- 

 acteristics of all the sponges (Fig. 22.2). Colonies of various species of 

 Leucosolenia grow just below the low tide mark. The body of each individual 

 is a sac whose open top is the excurrent opening or osculum. The current of 

 water that flows from this opening, carries particles outward and was the clue 

 by which Ellis in 1765 discovered that sponges are animals. Thousands of in- 

 current pores perforate the body wall, each one opening through a single pore 

 cell into the large central cavity or spongocoel (Figs. 22.2, 22.3). The outer 

 surface is covered with epithelial cells and flooded with mucous secretion that 

 hinders small animals from settling upon it. The central cavity is lined with 

 choanocytes or collared cells whose lashing flagella produce continual cur- 

 rents through the waterways of the sponge. Water enters through the incur- 

 rent pores bringing oxygen and microscopic particles of food with it. It passes 



Osculum 

 Pore 



Spicule 



Water, 

 food 



Spicule 



Water, food 



Outer covering 



Middle layer 

 of jelly 



B. 



inner layer 

 collared cells 



Fig. 22.2. A, a stage of a simple sponge with part of its wall cut away to reveal 

 the central cavity. This illustrates the fundamental characteristics of sponges. It is 

 a hollow vase with pores in its wall through which water and food enter a central 

 cavity. Water, waste and doubtless much food pass out through the main opening 

 (osculum). The intake and digestion of food is carried on by collared cells (choano- 

 cytes) that project into the cavity. B, a long section of the wall shows the lining of 

 the central cavity with its collared cells that catch particles of food, digest it, and 

 pass it on to the ameboid cells within the body wall. The spicules forming the 

 skeleton are each secreted by two cells that move inward from the outside layer. 

 The stage shown (known as Olynthus) occurs in the development of certain spicule- 

 bearing sponges. It is not a species. (Courtesy, Borradaile & Potts: The Inverte- 

 brata. Cambridge, England, The Macmillan Co., 1932.) 



