602 



NERVOUS SYSTEM. (COMPARATIVE ANATOMY.) 



Cuvier has also been unable to make out any 

 traces of a nervous system, and doubted the 

 accuracy of the statement made by Spix. These 

 animals are, however, extremely sensible to the 

 touch, when expanded, and to the light when 

 exposed to its influence. This would indicate 

 some degree of nervous sensibility, but which 

 we can conceive to be afforded by the nervous 

 elements being distributed in their homoge- 

 neous structure in a manner similar to the 

 preceding classes. In the next class, the 

 Acalepha, which consists of gelatinous marine 

 animals, Trembley,* Gcede,t Carus,J and other 

 anatomists have failed to detect any distinct 

 nervous filaments. Dr. Grant, however, de- 

 scribes what he considers a nervous system in 

 the Beroe pileus, and describes it as " consist- 

 ing of a double circular nervous filament, 

 situated around the oral extremity of the body, 

 which sends off minute filaments in each of 

 the spaces between the eight longitudinal bands 

 of ciliffi ; these eight points, from which the 

 longitudinal filaments come off, present minute 

 ganglionic enlargements." This statement has 

 been recently called in question, and it is pro- 

 bable that the nervous system in these animals 

 is diffused throughout the gelatinous mass of 

 which their bodies are composed. Dr. Milne 

 Edwards describes and figures part of the 

 nervous system in a larger species of Beroe 

 ( Lesiieura vitrea), as radiating from a single 

 small ganglion which is closely connected with 

 a coloured eye-speck, situated at the middle of 

 the superior extremity of the body.|| 



RADIATA. In the next group of animals, 

 the Radiata, nervous filaments are for the 

 first time discoverable ; and this being the 

 case, it is important that we should notice 

 what form and direction they assume : it is 

 that of a ray and a central point, or a nerve 

 and a ganglion ; of these several are de- 

 veloped ; and as it is the very essence of a 

 nervous system that it should consist of gan- 

 glions united and not separated, threads of 

 communication are developed, called commis- 

 sures, and a ganglionic system is formed, the 

 inferiority of which is expressed in the Echi- 

 nodermata by the perfect equality of all the 

 ganglions : these ganglions are also situated at 

 an equal distance from each other, and are 

 determined in their number and origin by the 

 general organization of the animals: thus we 

 shall find that in the Asterias, or star-fish, with 

 five rays, there are five ganglions (with radia- 

 ting nerves) sending off commissures, which, 

 inasmuch as they are situated on a spherical 

 surface, unite them in the form of a ring. 

 (See ECHINODERMATA, Jig. 23, p. 44, vol. ii.) 

 This ring we may call the primary nervous 

 ring; it is that form which we shall hereafter 

 recognize as the essential base of even the 



* Memoires pour servir a 1'histoire d'un genre 

 de polypes d'eau douce, 1774. 



t IJeitraege zur Anatomie und Physiologic der 

 Medusen, 1816. 



$ Anatomie Comparee, vol. i. 



Lectures on Comparative Anatomy. 



(I Ann. des Sc. Nat. n. s, t. xvi., and Owen's 

 Lectures, by White, p. 106. 



most varied forms of a nervous system. Tt is 

 only in the genus Asterias that a nervous system 

 has been distinctly seen ; and we are indebted 

 to Tiedemann for the first description of it, in 

 his Monograph of the Echinodermata.* In a 

 small species of this genus, it consisted of 

 a circular cord around the mouth, from 

 which proceeded a filament along each ray, 

 having, at its origin, a minute ganglionic en- 

 largement ; the nervous ring rested upon the 

 extreme edge of the central aperture in the cal- 

 careous frame-work of the body, and the fila- 

 ments rested on the inferior surface of the rays, 

 concealed by and at the base of the tubular 

 feet and suckers. Two other filaments, much 

 shorter than those just described, according to 

 Tiedemann,f are given off from each of these 

 ganglionic enlargements, to be distributed to 

 the stomach and other viscera. And Ehrenberg 

 affirms that the red points situated at the ex- 

 tremity of each ray are eyes, and receive nerves 

 connected with special ganglia. The statement, 

 however, has not received confirmation from 

 any subsequent observer; but Mr. E. Forbes 

 describes a kind of protective apparatus apper- 

 taining to these points, consisting of a peculiar 

 arrangement of the spines around them. [The 

 existence of ganglia is questioned by many 

 observers. Microscopic examination would 

 decide the point.] 



In the Echinus no nervous filaments have 

 hitherto been discovered ; but in the genus 

 Holothuria, Cuvier observes, " that there ap- 

 pears to be a very attenuated nervous cord 

 around the oesophagus."]; This Delle Chiaje 

 denies entirely. Dr. Grant, however, de- 

 scribes their nervous system to exist in the 

 form of a collar around the anterior part of 

 the body, giving off longitudinal filaments. || 



[It is remarkable that, although the nervous 

 system be very obscurely developed in these 

 animals, the action of the muscular integument 

 is extremely powerful. The slightest irritation 

 of the surface is sufficient to occasion forcible 

 contraction of the integument to such a degree 

 that the thin membranes of the cloaca become 

 lacerated, and large portions of the intestine 

 and other viscera are forced from the anal 

 aperture. " So common is this occurrence," 

 says Professor Rymer Jones, " that the older 

 anatomists were led to suppose that, by a 

 natural instinct, the animals, when seized, 

 vomited their own bowels. It is, in fact, 

 extremely difficult to obtain perfect specimens 

 of the Holothuridse from the constant occur- 

 rence of this accident."] 



We may next ask what are the characteristics 

 of an increase in developement of the primary 

 nervous rins: just mentioned, as the funda- 

 mental form of every nervous system. They 

 are precisely these : either that it is in itself 

 more highly developed, or that it is multiplied 

 and repeated several times. This we shall find 

 illustrated in nature ; the former in the Mol- 



* Anatomie der Rbhrenholothurie, &c. 1820. 



t Loc. cit. 



J Regne Animal, vol. iv. 



$ Memoir sull'asteria, &c. 1823. 



|| Lectures on Comparative Anatomy. 



