MAGOSPH^RA 



261 



sensory, the inner (Ent] having a digestive function. The 

 planula of a hydroid may be looked upon as a gastrula in 

 which the mouth has not yet appeared. 



What we want then is to be able to form some notion of 

 the steps by which the gastrula may be derived from a 

 unicellular form. There are two interesting organisms which 

 serve to indicate one way in which the gulf may be bridged. 



Sip 



Ent 



_ 



FIG. 63. A typical animal gastrula in vertical section, showing 

 ectoderm (Ekt), endoderm (Ent), enteron or digestive cavity (U), and 

 mouth (B //). (From Wiedersheim's Anatomy.) 



Haeckel has given the generic name of Magosphara 1 

 (Fig. 64, A, B) to a minute spherical organism about 

 0*07 mm. in diameter, consisting of a number of conical 

 cells radiating from a common centre and imbedded in 

 a clear gelatinous substance. Each cell has a nucleus and 

 contractile vacuole, and its outer or free surface is beset with 

 cilia by means of which the entire sphere is propelled through 



1 Unfortunately nobody has since seen this organism. 



