78 Invertebrata. 



2. Unjointed or obscurely segmented minute forms, 



with ciliated head-lobes = Class Rotatoria. 



3. Parasitic, flat-bodied forms, with no body cavity, 



and provided with suckers = Class Cotylidea, 



4. Parasitic forms with no suckers nor digestive 



canal, and with a hook-bearing proboscis = 

 Class Acanthocephala. 



5. Cylindrical, un jointed," non-ciliated forms, with 



digestive canal and body cavity, mostly para- 

 sitic = Class Nematelmia. 



6. Unsegmented forms with a proboscis, and convo- 



luted intestine, non-parasitic = Class Gephyrea, 



7. Segmented, bristle-clad worms with no suckers, 



moderate intestine, non-parasitic = Class 

 Chaetopoda. 



8. Segmented, unbristled, sucker-armed, external 



parasites = Class Hirudinea. 



9. Sessile, one-jointed, colony-building, rarely single, 



worms, living in cells and with a crown of 

 protrusible tentacles = Class Bryozoa. 

 10. Sessile or free, one-jointed worms, with one nerve 

 ganglion but no protrusible crown of tentacles 

 = Class Tunicata. 



CHAPTER XV. 



BRACK IOPODA, 



THE living representatives of this group were formerly 

 included among the Mollusca, but their anatomy and 

 the history of their development show that they are 

 constructed on a different plan. They were very 

 abundant at an earlier period of the world's history, 



