318 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



embryos of the higher forms, possess three germinal layers ecto- 

 derm, endoderm, and mesoderm. Alternation of generations is 

 exemplified in the life-history of Fluke, and possibly of Tape- 

 worm. 



5. Contractility. This is exhibited in the three forms of move- 

 ment, amoeboid, ciliary, and muscular. 



Amoeboid movement is typically seen in Amoeba, from which 

 animal its name is derived. It is also observable in the endoderm 

 cells of Hydra and the epithelial cells lining the intestine of the 

 Fluke. From this animal upwards the power of amoeboid move- 

 ment is mainly retained by the colourless corpuscles found in 

 lymph and blood (leucocytes). 



Ciliary movement is first seen in Yorticella. and is also shown 

 by the endodermal cells of Hydra, and the external surface of 

 the ciliated embryo in the Fluke. Cilia are found in the seg- 

 mental organs and oviducts of the Earthworm, and cover the 

 gills, labial palps, and inner side of the mantle-lobes in the Mussel, 

 in which animal they play a very important part in respiration 

 and nutrition, as is also the case in Amphioxus. In the Frog cilia 

 line the mouth-cavity, and are present in the lungs, oviducts, and 

 elsewhere. In the Pigeon and Eabbit ciliary action is mainly 

 exemplified in the trachea and oviducts. Most sperms are pro- 

 pelled by flagella. 



Muscular movement is foreshadowed in the cortical layer of 

 Vorticella and Gregarina, and tailed cells of the ectoderm in 

 Hydra. The muscles are at first largely connected with the skin, 

 as seen in the Fluke, Ascaris, Earthworm, and Leech. The 

 exoskeleton of the Crayfish, Mussel, and Snail, serves for the 

 attachment of muscles, as does the endoskeleton of the Dogfish, 

 Frog, Pigeon, and Eabbit. 



Muscular fibres play an important part in the working of most 

 of the organs of the body, and a distinction arises in the 

 vertebrate forms between unstriated and striated muscle, which 

 belong to the internal organs, and the muscles respectively. 

 These varieties of muscle fibre are distinguished by histological 

 characters (cf. p. 208), and their involuntary and voluntary nature, 

 the latter difference depending upon nerve-supply. 



Locomotion is one of the most important outcomes of con- 

 tractility. It is effected by amoeboid movement in Amoeba, 

 ciliary action in free-swimming Vorticellae, and muscular con- 



