72 



ZOOLOGY 



Within, the cavity of the corallites is divided into a series 

 of chambers by the presence of partitions termed tabulae ; 

 these may be simple flat plates, but more often they are cup- 

 shaped, and in some cases are drawn down in the centre into 

 funnel-shaped structures. The tabulae often give off tubes, 

 which run out into the platforms. 



The whole skeleton is so compact that it appears to be 

 formed of a continuous homogeneous deposit of calcium car- 

 bonate. In reality, however, it consists of a number of spicules, 

 each with minute serrations which fit into other serrations on 

 the neighbouring spicules. 



The polyp wall is built up of the ectoderm, mesogloea, and 

 endoderm. The mesogloea contains a few scattered cells and 

 fibres, as well as the skeletal spicules, which, however, in those 

 cases where the embryology is known, originate in certain ecto- 

 dermal cells which wander afterwards into the mesogloea. 



The tentacles, eight in number, have about fifteen pinnae 

 on each side in a single series. They are hollow, their cavities 



Fig. 51. — Transverse section of 

 a polyp of TuHpora purpurea. 

 After Hickson. 



1. Stomodaeum. 



2. Cilia of siplionoglyph. 



3 . Ectoderm of stomodaeum and 



of outer surface of body- 

 wall. 



4. Endoderm lining mesenteric 



chambers. 



5. Longitudinal muscle fibres in 



mesentery, cut across. 



6. Calcareous spicules. 



7. Parasitic diatoms on the 



ectoderm. 



8. Mesogloea. 



D. Dorsal surface. 

 V. Ventral surface. 



opening into the intermesenteric chambers, over which they 

 are placed. Their ectoderm is ciliated. The whole body of 

 the polyp, including the tentacles, can be withdrawn into the 

 corallite. The stomodaeum is ciliated throughout, and on one 



