Chap. III.] 



METAZOA. 



3 1 



} nr. Acineta 

 tuber osa. 



III. Suctoria : in these parasites (e.g. Acineta, 

 Fig. 3 in.), the month is lost and the sucking tubes 

 protruded from the protoplasmic 

 mass serve to convey food into 

 the body. A study of their 

 development reveals the interest- 

 ing fact that they commence life 

 as ciliated embryos, and suggests 

 the idea that they are descended 

 from ciliate infusoria. 



The Sporozoa will, for the 

 purposes of this book, be repre- 

 sented by the Gregarinida. The 

 forms best adapted for study are 

 the gigantic Gregarine found in 

 the intestine of the lobster, and 

 remarkable for being, though but 

 a single cell, as much as two -thirds of an inch in 

 length ; and the much smaller species found in the 



i testicular reservoirs of the 



earthworm. 



II. THE METAZOA. 



STRUCTURE AND EARLY HIS- 

 TORY OF THE EGG-CELL. 



The key to the structure 

 of the higher animals, or 

 Itletazoa, is to be found in 

 a knowledge of the early 

 history of the egg from 

 Avhich, as has been already- said, they all arise. 

 This cell, when mature, consists of a mass of proto- 

 plasm (Fig. 4, c), with a central nucleus (6), and con- 

 tained nucleolus, and in most, though not in all cases 

 (Hydra), it has a definite investing membrane (a). 

 Under normal circumstances this egg-cell is fertilised 



Fig. 4. Ripe Ovum of Cat. 

 (After Klein.) 



a, Envelope ; 6, nucleus ; c, 

 protoplasm. 



