PHYLUM PORIFERA. SIMPLE MULTICELLULAR ANIMALS 



95 



Dermd amoebocyte 

 Incurrent pore 



Food particle 

 Amoebocyte 

 Porocyte 

 Mesoglea 



Choanocyte 

 Central cavity 



Figure 47. Diagrammatic cross section, designed to show the cellular structure of the body 

 wall of a simple sponge {Leucosolenia) . Highly magnified. 



resembles in sbape a slender vase that bulges 

 slightly near the center. The osculum is sur- 

 rounded by a circlet of straight spicules, 

 and smaller spicules protrude from other 

 parts of its body. The body wall is perforated 

 by numerous incurrent pores. 



Scypha has one large central cavity 

 (spongocoel) (Fig. 48), which leads from 

 the base of the sponge up to the osculum 

 at the distal end. Around the central cavity, 

 the thick body wall is built up of elongated, 

 sack-shaped, flagellated chambers. Each of 

 these is perpendicular to the central cavity, 

 like the bristles on a bottle brush. The large 

 exhalent opening of each chamber (apo- 

 pyle) empties into the central cavity. These 

 chambers do not fit closely together; there 

 are narrow spaces between them. Water is 

 drawn into these spaces and then into the 

 chambers through many inhalent openings 

 (prosopyles) which abundantly pierce the 

 walls of the chambers. The flow of water 

 through the sponge is produced by the un- 

 correlated but constant beating of the 

 flagella of the collar cells (choanocytcs), 

 which more or less completely line the in- 

 side of each chamber. 



In the wall of the flagellated chambers 

 there occur (1) inhalent openings (pros- 

 opyles), (2) jellylike material called me- 

 soglea (mesenchyme), (3) spicules, and (4) 

 numerous amoeboid cells. The latter are of 

 three types: (1) pore cells surround pores 

 and mav close them in a muscularlike man- 

 ner; (2) scleroblasts manufacture spicules, 

 which are mineral skeletal structures abun- 

 dantly present; (3) archeocytes are embry- 

 onic amoebocytes with blunt pseudopodia, 

 which can produce other types of cells, par- 

 ticularly reproductive cells. 



The soft body wall is supported and pro- 

 tected by a skeleton consisting of a great 

 number of spicules composed of carbonate 

 of lime (Fig. 49). Four varieties of spicules 

 are always present: (1) long straight mon- 

 axon rods guarding the osculum, (2) short 

 straight monaxon rods surrounding the in- 

 current pores, (3) triradiatc spicules always 

 found embedded in the body wall, and (4) 

 T-shaped spicules lining the central cavity. 

 Spicules are formed within scleroblasts ( Fig. 

 49). A slender organic axial thread is first 

 built up within the cell; around this is de- 

 posited calcareous matter; the whole spicule 



