NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



607 



may be considered as a repetition of the other; 

 of these, the most anterior acquires the greatest 

 developement, and is called the head. So in 

 examining their nervous system, we shall find 

 that a primary nervous ring (formed of a gan- 

 glion and two semi-circular radiating nerves) is 

 contained in each segment. This ring, no 

 longer closed, as in the preceding classes, but 

 open, varies in degree of developement, accord- 

 ing as the segment which encloses it is in a 

 high or low degree of developement : thus, in 

 the cephalic segment, or head, we shall always 

 find developed a cerebral or supra-oesophageal 

 ganglion ; and, inasmuch as when a true ner- 

 vous system was first formed was first sepa- 

 rated from the punctiform homogeneous mass 

 of the gelatinous acrita commissures were 

 found uniting the primary masses of medullary 

 substance (as we saw in the Asterias), so ought 

 we to find, in the Articulata, commissures 

 uniting the primary nervous rings ; which latter 

 are now become longitudinal, and the com- 

 missures of the nervous ring itself are now 

 become radiating nerves. We ought also to 

 find that these commissures depend, in degree 

 of developement and in situation, on the same 

 characters of the primary nervous ring, and 

 consequently on the ganglia thereon developed. 

 We may next ask, what will mark the high 

 or low degree of organization of a nervous 

 system composed of several primary nervous 

 rings ? The researches of philosophical anatomy 

 inform us, that, first, a low degree will be 

 characterized by an undetermined number of 

 those rings by a nearly equal developement of 

 the whole, and by the central mass of nervous 

 matter accumulated on them being situated on 

 the ventral surface of the animal. Secondly, a 

 higher degree of organization exists when the 

 primary nervous rings are repeated in a deter- 

 minate manner when some of them predomi- 

 nate in developement over the others, and their 

 central medullary masses, or ganglions, are 

 situated on the dorsal aspect of the animal. 

 Again, as regards the uniting commissures, 

 these will, of course, depend, in degree of 

 developement, on the organization of the gan- 

 glions united by them ; and the more perfect 

 and the more intimate is the connexion esta- 

 blished by these commissures, the more highly 

 organized is the nervous system. 



In the Articulata about to be described, we 

 shall always find the most anterior nervous 

 ring developing a ganglion on its superior sur- 

 face a true cerebral ganglion. We shall find 

 this nervous ring repeated in the other segments 

 of the body, but in a much more imperfect 

 manner, for ganglions are developed only on 

 the ventral surface of the animal; and from 

 this latter circumstance, they, as well as their 

 commissures, cannot be highly developed. 



1 . Entozoa. In the lower forms of Entozoa, 

 as in the tsenia and cysticercus, no nervous 

 system is discoverable. These animals consist 

 of a gelatinous, more or less homogeneous 

 mass, in which no distinct nervous system 

 exists. In the Distoma hepaticum, the nervous 

 system consists, according to Bojanus,* of a 



* Isis, 1821, vol. i. p. 168. 



nervous collar or ring, with two lateral gan- 

 glions entwining the oesophagus, and two nerves 

 which are distributed on the posterior part of 

 the body. In the Ascaris lumbricoides, the 

 nervous system consists of a thin double fila- 

 ment, without ganglia, situated in the median 

 line of the abdomen, which separates to enclose 

 the opening of the vulva, and to encompass 

 the oesophagus at the lower part of the mouth. 

 In the Strongylus gigas, according to Otto,* 

 the median nervous filament consists of very 

 closely approximated ganglia, thus advancing 

 a step higher in organization, and approaching 

 to the character of the true articulated classes. 



2. Rolifera.- The ilotifera are minute mi- 

 croscopic animals : in them Ehrenberg has 

 discovered and described a rather complex ner- 

 vous organization, sufficiently so to justify their 

 being ranked thus high in the scale of animated 

 beings.f In the llydatina senta, according to 

 this anatomist, the nervous system consists of 

 two closely approximated filaments running 

 along the abdomen, and giving off lateral 

 branches in their course forwards : arrived at 

 the anterior part of the body, these nerves form 

 a large ganglion, and then ascend to embrace 

 the oesophagus in the form of a ring, on which 

 minute ganglia are developed, giving off nu- 

 merous filaments to the surrounding parts. 

 There are four of these lateral ganglia, besides 

 the large supra-oesophageal ganglion. 



3. Cirrhopoda. In the Cirrhopoda, the 

 abdominal nervous cords have regular ganglia 

 developed on them, and there is a nervous 

 collar round the oesophagus, as in the preceding 

 classes. Cuvier observes,]: that in a species of 

 Lepas he found two nervous cords situated on 

 the ventral surface of the body, with five double 

 ganglia developed on them, from which were 

 given oft' lateral filaments to supply the curled 

 feet. Anteriorly, and at the lower part of the 

 mouth, these cords separated more widely, to 

 encircle the oesophagus, above which they de- 

 veloped a quadnlobate ganglion, from which 

 were given off four nerves to the viscera and 

 muscles. 



4. Annelida. The nervous system of the 

 Annelida consists of a varied number of ganglia, 

 united by double longitudinal commissures, 

 running along the ventral surface of the body, 

 from which lateral filaments are given off. 

 There is also a supra-oesophageal ganglion, 

 which, being connected by lateral nervous cords 

 with the first pair of infra-oesophageal ganglia, 

 form a ring or collar, surrounding the oesopha- 

 gus : this we at once recognize as the most 

 anterior of the column of primary nervous 

 rings, with the ganglion developed on its supe- 

 rior surface. I have examined the nervous 

 system in the genera Lumbricus, Aphrodita, 

 and Hirudo; the general plan was the same in 

 all. In the Lumbricus terrestris, or common 

 earth-worm, a nervous cord passed along the 

 whole ventral surface of the animal, and pre- 

 sented, in a small species, the appearance 



* Berliner Magazin, 1814, p. 178. 

 t Organisation Systematik cler Infusions-Thier- 

 chen, Berlin, 1830. 



} Anat. des Mollusques. 



