LOWER INVERTEBRATES. In the Aletazna the nervous system develops in the ectoderm or outer• layer of the body wall. which is the region that comes in contact with the outside world and hence is the one in which we should expect sensi tiveness to become first developed and localized. Some of the emlenterates have in the ectoderm superficial sensory cells with which the nerve cells are in contact. Fibres from the nerve cells connect in turn with the fibrillar part of the muscle cells. In the flatworms the uer•ois sys tem is almost completely separated from the epithelium and lies in or close under the dermal musculature, and a 'brain' is best developed in some of the Polyeladida and some trematodes. The nervous system of eehinoderms has a devel opment unknown in other animals. It is com posed of three independent systems: (I) the su perficial oral; (2) the apical oral: (3) the deeper oral. The superficial oral consists of ring around the o‘sophag.us, and of radial nerves which branch out from this ring to the arms and radii. This system innervates the intestine. ow buhu•rad feet, and integument. The nervous sys tens of Mollusca is of a complex type of its own, but shows some affinities to that of annelids and arthropods. In its simplest form (ehitons) it consists of a cerebral part, from which four pairs of commissures are sent off. Passing ventrally from it there is a ring with ganglia, and passing backward two pairs of commissures. one called `pedal' and the other 'pallial! All of these con tain ganglion cells along their whole length. and send off various branches and commissures. The pallial commissures are united posteriorly. In the Pckey podia the nervous system is somewhat more complex, but it is always symmetrical, and consist: of two ganglia. the one On each side of the gullet, and united above by a commissure. Each ganglion gives a nerve cord, which passes 110'1{11 ward and backward to the bilobed pedal ganglion of the foot. Directly backward from the cerebra] ganglia also proceed two dither cords to the bilobed. fused, viscera]
ganglion. situated can the ventral side of the pos terior adductor muscle: this ganglion sends out branches to the palps and mantle. The pedal ganglion innervates the foot. the viscera, the enteric canal, the heart, the gills, and a part of the mantle. The nervous system of gastropods varies in the different groups both in regard to the arrangement of the ganglia and the There is usually a pair of ganglia near together or some distance apart. which lie above the gullet. and give off the cords behind. The viscera] cords pass eventually into a pair of ganglia. but only after they have passed through the pleural ganglia. Pedal and buccal ganglia with their commissures also exist.
The nervous system of annelid: is well devel oped and has a bilateral and metamerie arrange ment. It consists of the bilobed cerebral ganglion or brain and of a double ventral chain of ganglia and commissures. The brain lies in the prosto mium or a little farther back, as in the ease of the earthworm. Branches are given off from it to the eyes, tentacles, and sensory epithelium. Two lateral commissures from it pass ventrally around the alimentary canal and unite below it into the double ventral nerve chain. One ganglion occurs in this nerve ehain for each segment. In some forms the two halves of the chain are widely separated and are united by transverse commis l'Pq. The arrangement of the nervous system of arthropods is essentially like that of annelids. There is a suprmesophageal ganglion or brain that gives off commissures which pass downward and backward to unite with the first ganglion of the ventral nerve chain. Various degrees of centration of the nervous system oceur through out the Cm-taco:1 and the insects. The higher Crust:welt, as well as the insects, have a visceral nervous system. The brain of insects is rela tively large and is lobed.