48 EVERS'S COMPARATIVE ANATOMY. 



acaiephae and echinodermata, fine nervous filaments and small 

 white, ganglions surround the entrance of the alimentary canal. 



Dlplo-neura, Grant.— In this great division, the nervous 

 system, presents the same extended form as the body, placed on the 

 ventral surface of the alimentary canal, and except in the higher 

 classes, not enclosed in an osseous sheath. Among the higher 

 forms of the entozoa, as the ascaris, two fine nervous filaments 

 extend along the median line of the abdomen, separating to embrace 

 the oesophagus, and the vulva of the female. In the notommata 

 clavulata of the rotifera, we find nine pairs of ganglions disposed 

 along the course of the lateral columns. Scarcely a trace of ner- 

 vous system can be perceived in the simple forms of the annelida. 

 In the nereids, however, and many others of this class, the sympa- 

 thetics become quite distinct ; and numerous nerves are seen to pass 

 off in the lateral direction. The common leech, which presents 

 about eighty rings in the trunk of its body, has five and twenty 

 ganglia placed along: the abdomen, approximated at the two extre- 

 mities of the column. In the most inferior of the diversified class 

 of the crustaceans, the nervous system presents itself in the form of 

 two slender abdominal filaments, in imitation of what we have seen 

 in the preceding classes, and by a gradual development from the 

 peripheral to the central parts, it arrives at that concentration of 

 nervous ganglia around the oesophagus, which connects the highest 

 of the articulate with the molluscous classes. 



' Cyclo-gangliata.— The greater number of the mollsuca being 

 aquatic, their nerves present the same pale and soft characters 

 observed in the other aquatic invertebrates ; hence the difficulty of 

 indicating their particular distributions. Here as in the radiata, the 

 same tendency to accumulate nerves around the entrance to the 

 alimentary canal prevails, but in this case more generally accom- 

 panied with ganglia. In the lowest classes of the division, as the 

 tunicata and conchifera, the nervous chords are placed beneath the 

 alimentary canal ; in the two next classes gasteropoda and ptero- 

 poda, they are more in the vicinity of the stomach ; arid in the ce- 

 phalopoda, which is the last and highest of the division, the nervous 

 ganglia attain a more elevated position, they cease to embrace the 

 oesophagus; and a distinct brain, as in the vertebrata, with nume- 

 rous symmetrical ganglia along the abdomen take, their place. 



CHAPTER VIII. 



NERVOUS SYSTEM IN THE VERTEBRATA. 



PISCES. ^ 



We no longer find the nervous system perforated by the alimen- 

 tary tract. On, the contrary, in all the succeeding classes, it 



