AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



nerve-fibres. The latter are especially characteristic of the 

 sympathetic system. 



There is reason to believe that the nervous system is derived from the 

 epidermis, with which, in lower forms, it often remains closely connected. 

 The deeper position in higher forms has been assumed for protective 

 purposes. The development of the brain and spinal cord (see Frog) exem- 

 plifies the steps in such a change of position. A nervous plexus underlying 

 the epidermis appears to have preceded definite nerve-cords, which have 

 probably arisen by condensation of parts of such a plexus, with which they 

 may co-exist. In Amphioxus, for example, there is such a plexus. 



The typical form of the nervous system (when differentiated nerves are 

 present) in a radially symmetrical animal, is that of a ring. The assump- 

 tion of bilateral symmetry would pull this out into a long loop, the anterior 

 end of which might thicken into cerebral ganglia. The sides of the loop 

 might remain separate, and even (cf. lateral nerves of Fluke) become dis- 

 connected behind. By the ventral fusion of such lateral nerves a circum- 

 esophageal ring and double ventral cord (Earthworm and Crayfish) might 

 be produced. 



(2) Sense Organs. These are not differentiated in Amoeba, 

 Vorticella, and Gregarina, and are absent in Tapeworm as a 

 result of parasitism. It may be mentioned here that a sense 

 organ essentially consists of one or more usually elongated sense- 

 cells (end-organs), of epithelial nature, and connected on the one 

 hand with the nervous system, while on the other they are 

 adapted to receive impressions from various stimuli coming, in 

 the large majority of cases, from the exterior. 



The function of the lateral line organs and ampullae of Dog- 

 fish are not certainly known. 



(a) Tactile Organs. Under this heading may be classified with 

 more or less certainty cnidocils and palpocils of Hydra, head- 

 papilla of Fluke, tactile papillae of Ascaris, segmental papillae of 

 Leech, setae of Crayfish, labial palps and tentacles round inhalent 

 opening in Mussel. Definite tactile cells are present in the skin 

 of Snail, Amphioxus, Dogfish, Frog, Pigeon, and Rabbit. 



(b) Gustatory Organs. These are not certainly known except 

 in the Frog, Pigeon, and Rabbit, where, as taste-cells, they are 

 supplied by the glosso-pharyngeal nerve. In the last case they 

 occur aggregated into well-marked taste-buds, localized in the 

 circumvallate papillae and papillae foliatae. 



(c) Olfactory Organs. These are probably represented in the 

 Crayfish by olfactory setae, and in the Snail by the epithelium 

 on the tips of the tentacles. The Dogfish, Frog, Pigeon, and 



