178 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



Among the Metazoa sucking tubes are best developed 

 in the Rhizocephala; these parasitic Crustaceans begin 

 life in the shape of free-swimming naupliiform larvae 

 (see page 534), attach themselves to the bodies of 

 higher Crustacea, and, losing all appearance of having 

 true appendages, develop at their anterior end a 



Fig. 80. Sacculina, carcini. A, Adult form showing the tuft of roots 

 which it insert* into the boJy of its host. B, The naupliiform larva. 

 (After Haeckel.) 



number of filamentar processes. These make their 

 way into the body of the host, and by endosmosis 

 take up nutriment, which they pass on to the shapeless 

 body. 



The mouth and the commencement of the diges- 

 tive tract are often adapted for sucking, as in the 

 liver-fluke, where the contractions of the protractor 

 and retractor muscles of the pharynx effect this pur- 

 pose ; in the JVematoid worms, where, as in the 

 fluke, the intestine is complete, and is, moreover, 

 provided with an anus, muscles set radially around 

 the oesophagus extend to the walls of the body ; 



