PHYSIOLOGY OF THE INVERTEBRATA. 387 



3Iartens,* FlanaHa liclienoidoi moves by means of the pro- 

 truded lobes of its muscular pharynx. 



The Hot if era. — The muscular system of these animals is 

 composed of unstriated fibres. The most characteristic ap- 

 paratus is the so-called " wheel." By its agency these 

 animals swim freely about, or, when at rest, create certain 

 water currents. The " wheel " or trochal disc and its append- 

 ages vary in different genera. The edge of this disc is 

 generally ciliated, but in some forms (('.//., Stcj^Jianoccws) it 

 is produced into ciliated tentacula. Besides being organs of 

 locomotion, the appendages of the trochal disc are indirectly 

 prehensile organs. 



The Trcmntofla and Cedoidea. — The movements of the 

 body are due to sucking-cups and cavities (i.e., suctorial 

 organs), horny hooks and spines. 



The Annelida. 



Muscular tissues are highly developed in the Annelida. 

 In Jfirudo, the muscular system into which the integument 

 is continued, forming a dermo-muscular tube, consists ex- 

 ternally of a circular muscular layer, and internally of a 

 longitudinal muscular layer. Both these layers are traversed 

 by radial muscle-fibres, which run from the interior of tlir 

 body to the surface. At the lateral edges of the body, the 

 radial muscles pass directly from the dorsal to the ventral 

 sm'face. Certain muscle-fibres run obliquely. In Hlrudo, 

 locomotion is chiefly effected by means of the suckers, which 

 contain both circular and radiating muscle-fibres. The 

 posterior sucker is attached to something, then the animal 

 stretches itself forward to its fullest extent and fixes its 

 anterior sucker. After releasing the posterior sucker the 

 body is powerfully contracted. The posterior sucker now 

 attaches itself close to the anterior sucker, which is then 

 loosened and thrust forward as before. Hirudo can also 



* Mimoires de VAcadimie Impiriale des Sciences de St. Pitersbourg, tome 2. 



