CCELENTERATA — CORALS 



125 



each side, and it is through their expansion and contraction, 

 thus admitting and expelling water, that the animal is able to 

 expand and contract. When the animal is disturbed these 

 muscles contract, the tentacles are rolled inward and the entire 

 body becomes a small dome-like mass. 



There are no sense organs present, excepting specialized tac- 

 tile cells. There are many nerves present in both ectoderm and 

 endoderm which regulate the complexity of the animal's move- 

 ments. 



Astrangia reproduces by development of spermatozoa and 

 ova. The sexes are distinct. The reproductive elements form 

 by the division of some of the endoderm cells of the mesenteries 

 near the lower end of the body. The product of the union of 

 an ovum and spermatozoon develops rapidly into an ovoid, 

 hollow body, the planula (blastula stage). This elongate body 

 has two layers of cells, an inner (endoderm) and an outer ciliated 

 layer (ectoderm). After swimming about for a time by means 

 of its cilia, the planula settles down to the bottom of the water 

 and becomes attached. At the free end an inbending of the wall 

 occurs, forming mouth and oesophagus and finally piercing the 

 inner cavity (gastric cavity). 



Secretion of lime. — After the larva sinks down from its 

 free-swimming condition and rests upon some object, certain 

 ectoderm cells (calicoblasts) of the base of the animal begin 

 to secrete a calcareous liquid directly exterior . to themselves, 

 and from this is inorganically crystallized the calcium car- 

 bonate (i8) (Fig. 47). According to some students the " cal- 

 careous deposit is laid down within the individual calicoblasts 

 of the ectoderm. At the same time new ectodermal cells are 

 formed next the mesogloea, and these which are undergoing 

 calcification become loose external layers of partly calcareous, 

 partly organic tissue." Ogilvie. 



This deposit, developed between the base of the animal and 

 the rock or other foreign object to which it is attached, grad- 

 ually forms a calcium carbonate base (called the basal plate) 



