48 THE POPULAR SCIEXCE MONTHLY. 



continuous lines, and called nerve-trunks^ or simply nerves. The gan- 

 glia are the centres of poiner. The trunks are the lines of communi- 

 cation between the ganglia and the different parts of the organism. 



Without descending into the debatable ground between the vege- 

 table and animal world (for, strange to say, the boundary-line between 

 tAVO modes of life a})parently so difterent has never yet been estab- 

 lished), let us examine an organism as lowly as any possessing a dis- 

 tinct and single nervous system an ascidian moUusk Avhich simply 

 means a bag-shaped, soft-bodied animal. It has no head, nor any 



Fig. 1. Nervous Ststem op Ascidian : 1. Mouth; 2. Vent; 3. Ganglion. 



organs of sight, hearing, or smell. It consists of a sac, from the lower 

 portion of which proceeds a stomach or digestive tube ; these are cov- 

 ered with a muscular envelope, and the whole is inclosed in another 

 envelope or membranous sac called a mantle. These coverings are 

 pierced by two openings a mouth for admitting water and the nu- 

 trient particles which it happens to contain into the inner sac or com- 

 mon reservoir, which also serves as a respiratory organ and a vent, 

 communicating directly with this sac, and also with the more cii'cui- 

 tous digestive tube. A constant stream of Avater passes through the 

 mouth into this common or respiratory sac, where, after having served 

 its purpose of supplying oxygen to the system, a part is ejected di- 

 rectly through the vent, and the remainder, together with the nutrient 

 particles, passes into the digestive tube, and thus finds its exit. 



The nerA^ous system, w^hich is the part most important to our pres- 

 ent purpose, is of the most simple kind ; it consists of a single gan- 

 glion, situated between the two orifices of the body, Avith each of 

 Avhich it has lines of nerve-communication, and also Avith the various 

 envelopes Avhich mainly constitute the animal. 



All the creature's moA'^ements must be carried on bv means of this 

 simple nerA'ous arrangement ; and, as it is fixed to one spot during its 

 whole life, they must necessarily be of a A'ery limited character ; it 

 has, in fact, but one moA'ement it contracts when touched.' Sup- 

 pose, for instance, some offending substance to have found its Avay into 

 the common sac, the irritation caused by it is transferred along the 



' For many of the facts and illustrations here stated, see '' Mental Physiology," by 

 Dr. Carpenter, chapter ii. 



