COMPARATIVE ANIMAL MORPHOLOGY AND PHYSIOLOGY. 319 



tractions in the higher forms. The Earthworm progresses in 

 a characteristic fashion, by contractions of its dermal musculature, 

 while Amphioxus and Dogfish can swim by means of their 

 lateral muscles, aided in the latter by paired fins. The tadpole 

 swims pretty much like a fish, but the Frog is adapted to 

 terrestrial locomotion by means of transversely jointed limbs, 

 which are also effective swimming organs. The limbs of the 

 Rabbit are completely given up to movements on land, and in it, 

 as in the Frog, the hind-limbs are the longer, this being 

 connected with the leaping habit. In the Pigeon the fore-limbs 

 are modified into organs of flight. 



6. Irritability and Spontaneity (1) Nervous System. Un- 

 differentiated in Amreba, Vorticella, and Gregarina, and very 

 diffuse in Hydra. The Fluke and Ascaris possess a fairly well- 

 developed nervous system with ganglia and nerve-cords. The 

 two elements, ganglion-cells and nerve-fibres, are here found, 

 the former being mainly confined to the ganglia. In the Earth- 

 worm the segmentally arranged nervous system consists of a 

 nerve-collar and ganglionated ventral cord, but the ganglia, with 

 the exception of the cerebral, are not very distinct, and the 

 ganglion-cells are not confined to them. Nerve-fibres are afferent 

 and efferent, the former being mainly sensory, the latter mainly 

 motor. The nervous system of the Leech is similar, but the 

 ganglia are very distinct, and this is also the case in the Crayfish, 

 where, however, a good deal of fusion has taken place, especially 

 anteriorly. At the same time a distinct head, containing the 

 main ganglionic masses and with which the chief sense organs 

 are connected, is present. The nervous system of the Mussel is 

 made up of three pairs of widely separated ganglia connected by 

 nerve-cords. Three pairs can also be recognized in the Snail, but 

 here they are very much localized and a distinct head is present. 

 Amphioxus possesses a dorsal nerve-tube contained in a neural 

 canal. The Dogfish, Frog, Pigeon, and Rabbit possess very 

 complicated nervous systems, consisting of brain with spinal cord, 

 contained in the neural canal, cranio-spinal nerves, and sympa- 

 thetic system. The brain becomes more complicated, mainly by 

 the increase in size of the cerebral hemispheres and cerebellum, 

 the former of which are connected together in the Rabbit by the 

 corpus callosum and fornix. At the same time, histological 

 distinction may be drawn between medullated and non-medullated 



