172 THE MICROSCOPIST. 



FAMILY 3. Acanthocephala. Body flattened, transversely 

 wrinkled ; sexual organs in separate individuals. 



FAMILY 4. Gordiar.ea (Hairworms). Body filamentous, 

 cylindrical ; alimentary canal present ; sexes distinct. 



FAMILY 5. Protozoidea or Gregarinida. Probably larval 

 forms. 



ORDER II. C^LELMINTHA. Alimentary canal distinct. 



FAMILY 1. Nematoidea (Roundworms). Body cylindri- 

 cal, hollow ; sexes separate. 



The Enoplidce tribe is distinguished by an armature of 

 hooks or styles round the mouth. Most of them are 

 microscopic. 



XY. ANNULATA (Red-blooded Worms). Some of these, 

 as the Serpula, etc., are inclosed in tubes formed of a shelly 

 secretion, or built up of grains of sand, etc., agglutinated 

 together. Many have special respiratory appendages to 

 their heads, in which the microscope will exhibit the cir- 

 culation. The worms of the Nais tribe, also, are so trans- 

 parent as to be peculiarly fitted for microscopic study of 

 structure. The dental apparatus of the leech consists of 

 a triangular aperture in a sucking disk, furnished with 

 three semicircular horny plates, each bordered with a row 

 of eighty to ninety teeth, which act like a saw. 



ORDER 1. TURBELLARIA. Body bilateral, soft, covered 

 with vibratile cilia, not segmented ; eyes distinct ; sexless 

 or hermaphrodite. 



ORDER 2. SUCTORIA (Apoda). Body elongate, ringed, 

 without bristles or foot-like tubercles; locomotion by 

 sucking-disks ; no external branchiae. 



ORDER 3. SETIGRADA (Choetopoda). Body ringed, elon- 

 gate, with feet or setigerous rudiments of them ; external 

 branchise usually present. 



XVI. CRUSTACEA. In the family of Isopoda the micros- 

 copist will find the Ascellus vulgaris, or water wood-louse, 

 of great interest, as readily exhibiting the dorsal vessel 

 and circulating fluids. 



