NERVOUS SYSTEM. (COMPARATIVE ANATOMY,) 



603 



lusca, the latter in the Articulata. In the 

 Mollusca, what is it that constitutes an increase 

 in developement of the primary nervous ring, 

 the characteristic form of the nervous system 

 of that class ? 



1. The greater volume of a central medul- 

 lary mass, and its situation on the dorsal aspect 

 of the animal. 



2. A small number of ganglions in the pri- 

 mary nervous ring, proportioned to the deve- 

 lopement of the muscular system, one predo- 

 minating in size over the rest, especially if that 

 one be situated on the dorsal aspect of the 

 animal. These ganglia are disposed unsym- 

 metrically throughout the body, whence Pro- 

 fessor Owen has designated these animals, in 

 reference to their nervous system, Heterogan- 

 gliata. 



MOLLUSCA. 1. Tunicata. Many of the 

 Tunicata, the lowest of them, approach in cha- 

 racter the Zoophytes ; for no particular medul- 

 lary mass constituting a nervous system is dis- 

 coverable in the soft texture of their bodies, 

 except in but few of the genera, principally in 

 the forms of Ascidise. In the Ascidia mam- 

 millala (Jig. 336), Cuvier describes and figures 

 the nervous system* as consisting of a single 

 oblong ganglion, situated near the anus of the 

 animal, and between that and the branchial 



Fig. 336. 



Ascidia Mammillata, (after Cuvier,) shewing the 

 single ganglion between the branchial and anal 

 orifices. 



* Anatomic des Mollusques. 



orifice ; from this ganglion branches are given 

 off, some of which, passing to the oesophagus, 

 encompass it in the form of a ring. 



[Mr. Garner, in his valuable paper on the 

 nervous system of molluscous animals, de- 

 scribes the nervous system of Phuliusia intes- 

 tintilis. The single yellowish ganglion lies 

 upon the muscular coat between the two ori- 

 fices. One set of filaments coining from it 

 surrounds the branchial orifice, and gives nerves 

 to its tentacula, appearing to meet on the oppo- 

 site side, forming a nerve which seems to run 

 along the edge of the elongated branchial fold. 

 The other set supply the muscular tunic and 

 go towards the mouth. In Cynthia and those 

 Utnicata that have thick muscular tunics, the 

 ganglion is not visible external to the muscular 

 sac, it being situated in its interior. 



As the actions of these animals are ex- 

 tremely simple, so is their nervous system : by 

 the branchial orifice water is drawn in to supply 

 the branchiae and to convey nutrient matters 

 to the mouth. It is propelled by the action of 

 numerous cilia which cover the surfaces with 

 which it comes in contact. Through the anal 

 orifice are expelled both the current which sup- 

 plied the respiratory surface and that which 

 passes through the digestive canal. Each ori- 

 fice is provided with a sphincter muscle, which 

 may oppose the entrance of various matters at 

 one orifice, or resist their exit at the other. These 

 muscles receive filaments from the ganglion. 

 The animal is surrounded by a muscular sac, 

 which by its contraction can compress and 

 empty its general cavity. This, too, receives 

 some nervous filaments. The solitary ganglion 

 of this ascidian seems to regulate the actions 

 of its orifices of ingestion and egestion, and of 

 its enveloping sac on which depends the slight 

 locomotive power of the free species. We are 

 not prepared to deny to even this simple being 

 that prevailing attribute of animals, a will, and 

 therefore it may be assumed that its actions are 

 partly volitional and partly reflex (mental and 

 physical) while some are, no doubt, due 

 simply to the inherent irritability of its muscu- 

 lar tunic. 



2. Conchifera. In this order the nervous 

 system is precisely adapted to the functions these 

 animals have to perform. These are ingestion 

 of the food, respiration, and locomotion. Their 

 nervous centres or ganglia are, consequently, 

 placed in immediate relation to the organs 

 destined to those functions ; but as one pair 

 communicate with the others, it may be pre- 

 sumed to exercise an influence over them, and 

 to be the principal centre, the analogue of the 

 brain in the higher animals, the focus of sen- 

 sation and volition. 



The ossophageal or labial ganglia are for this 

 reason the most important. They are two in 

 number; they are situated more or less near 

 the mouth, and are united by a transverse branch 

 which arches over it. From these ganglia 

 nerves are given off to the mouth, and to the 

 tentacles, and to the anterior parts of the vis- 

 cera. Each ganglion has a branch of commu- 

 nication to the pedal ganglion and also to the 

 branchial ganglion. 



