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 epidermal débris 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 sometimes establish themselves on fishes, and, if there are no scales of the epidermis which annoy them, there are mucosities which are incessantly 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 Riciniœ, 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 Tœnia cucumerina, a tapeworm common to dogs. The cock, the turkey, and the peacock, carry each a distinct species of Riciniœ, and oftentimes several species are found on a single bird. Fig. 6 represents a form which infests the pygarg or sea-eagle.
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| Fig. 5.—Ophiodendrum Abietinum on Sertularia abietina. |
Fig. 6.—Ricinus of the Pygarg. | |
Fishes harbor crustaceans instead of insects, frequently in enormous numbers. 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 appearance in the course of their lives, and, although permanent tenants when once established, they retain their fishing-tackle and locomotive apparatus. The greater number of osseous fishes lodge Caligi on the
