312 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



conditions, no special modification for this purpose, but in Vorti- 

 cella and Gregarina a firm cuticle is present. All three pass 

 through an encysted stage. A cuticle is also found in Tapeworm, 

 Ascaris, Leech, and Earthworm, and to a more marked extent in the 

 Fluke, where it is much thickened and provided with numerous 

 minute spines. The maximum of cutictilar protection is reached 

 in the Crayfish, Mussel, and Snail, where a firm exoskelelon is 

 produced by the impregnation of a thickened and laminated 

 cuticle with salts of lime. 



In Hydra the thread-cells serve as a very effective protection. 

 The skin of the Dogfish, Frog, Pigeon, and Rabbit serves for 

 protection, and respectively possesses scales, poison glands, 

 feathers, and hairs. The colour of the skin is protective in 

 Frog and Kabbit. From the Fluke upwards, support is 

 largely afforded by what may be termed indifferent tissues, 

 i.e., connective tissue, and its modifications, cartilage and bone. 

 Cartilage first appears in the odontophore of the Snail, and, 

 together with bone, .forms in the Dogfish, Frog, Pigeon, and 

 Eabbit a more or less complete endoskeleton. The most primitive 

 part of this endoskeleton appears to be the backbone, the founda- 

 tion of which is the notocJwrd which, in Amphioxus, persists 

 without modification, except that it extends further forwards 

 than in higher animals. In the Chordate forms, the body is 

 divisible into an upper, smaller, neural tube, contained in the 

 skull and backbone, and a lower larger visceral tube. 



2. Nutrition. The food of Amoeba can be ingested, and refuse 

 egested at any point of the shapeless body, and the pseudopodia 

 are used for overwhelming any minute organisms that may serve 

 as nutriment. Vorticella is usually fixed, and the place of pseudo- 

 podia is taken by cilia, which do not themselves seize food, but 

 cause food-bearing currents to enter the vestibule and gullet. 

 Within the body the food takes a definite course, and the undi- 

 gested parts are cast out at a special anal area. The necessity 

 for definite points at which food may be taken in, and innutri- 

 tions residues ejected is brought about by the presence of the 

 firm cuticle. In Gregarina, however, the ready prepared food 

 is absorbed through the cuticle. 



Hydra presents a great advance upon this state of things. 

 The tentacles can seize small animals (which are first paralyzed 

 by the thread-cells) and convey them to the " mouth." Within 



