THE MICROSCOPE IN ZOOLOGY. 171 



but to the microscopist the palate, or tongue as it is called 

 a tube which passes beneath the mouth, opening ob- 

 liquely in front, and which is covered with transverse 

 rows of minute teeth set upon plates presents characters 

 of great value in classification. These palates require 

 careful dissection, and when mounted in balsam become 

 beautiful polariscope objects (Plate XVI, Fig. 129). 



XIII. CEPHALOPODA. The crystalline lens in the eye of 

 the cuttle-fish is said to be of the same form as the well- 

 known " Coddington lens." The skin of this class con- 

 tains a curious provision for changing its hue, consisting 

 of large pigment-cells containing coloring matter of vari- 

 ous tints. 



The suckers, or prehensile disks, on the arms of cephal- 

 opods often make interesting opaque objects when dried. 



XIV. ENTOZOA. These are parasitic animals belonging 

 to the class of worms. They are characterized by the 

 absence or low development of the nutritive system, and 

 the extraordinary development of their reproductive or- 

 gans. Thus the Tvenia or tapeworm has neither mouth 

 nor stomach, the so-called " head " being merely an organ 

 for attachment, while each segment of the " body " con- 

 tains repetitions of a complex generative apparatus. 



Among the Kematoid or round worms, the Anguilhdaz, 

 or little eel-like worms, found in sour paste, vinegar, etc., 

 as well as the Trichina spir.alis, inhabiting the voluntary 

 muscles, are generally classified. 



ORDER I. STERELMINTHA. Alimentary canal absent or 

 indistinct. 



FAMILY 1. Cestoidea. Tapeworms; body strap-shaped, 

 divided into transverse joints ; alimentary canal indistinct. 

 The cystic Entozoa (Echinococcus, etc.) are nurse or larval 

 forms of Cestoidea. 



FAMILY 2. Trematoda. Body mostly flattened ; alimen- 

 tary canal distinct ; branched. 



