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THE POPULAR SCIENCE MONTHLY. 



tained. Many insects shelter themselves in the fur of the mammalia 

 or in the down of birds, and remove from the hair or the feathers the 

 pellicle and cjoidermal debris which encumber them. At the same 

 time they minister to the outward appearance of their host, and are 

 of great use to him in a hygienic point of view. Animals living in 

 the water are similarly served by minute crustaceans. These some- 

 times establish themselves on fishes, and, if there are no scales of the 

 epidermis which annoy them, there are mucosities which are inces- 

 santly renewed in order to protect the skin from the continual action 

 of the water. Among the insects found on the skins of mammals and 

 birds that yield some return for the hospitality they receive, those 

 belonging to the family Hicinice, and commonly known as ticks, are 

 very numerous. Among the many generic divisions, one of the most 

 interesting has received the name of Trichodectes ,' it contains twenty 

 species, one of which lives on the dog, another on the cat, another on 

 the ox ; in a word, there is a distinct species on each of the domestic 

 mammals. The species infesting the dog has lately attracted especial 

 attention, from the circumstance that it lodges the larva of the Taenia 

 cucumerina, a tapeworm common to dogs. The cock, the turkey, and 

 the peacock, carry each a distinct species of Ricinia?, and oftentimes 

 several species are found on a single bird. Fig. 6 represents a form 

 which infests the pygarg or sea-eagle. 



Fig. 5. Ophiodendrum Abietinum on 

 Sertularia abietina. 



Fig. 6. RiciNue op thb Ptqabg. 



Fishes harbor crustaceans instead of insects, frequently in enor- 

 mous numbei'S. They live on the produce of cutaneous secretions, and 

 thus, like the ticks, are of service to their hosts. The Caligi and 

 Arguli, known usually as fish-lice, are among the most common of 

 these, and both are elegant forms, that change but little in appear- 

 ance in the course of their lives, and, although permanent tenants when 

 once established, they retain their fishing-tackle and locomotive ap- 

 paratus. The greater number of osseous fishes lodge Caligi on the 



