28 



which arc adapted to serve as food, are retained and digested 

 ill the stomach. Even these animals, fixed to one spot 

 during all but the early part of their lives, and presenting but 

 very slight indications of sensibilitj^, possess a regular heart and 

 system of vessels ; and these vessels form part of the stem, by 

 vrhich the compound species are connected. — Dr. Carpenter, 

 speaking of the nervous system of these animals, remarks : — 

 " The simplest form of the nervous system in this class 

 is seen in the solitary Tunicata, and Ascidia. Betvreen the 

 orifice by which the vrater enters and that by vrhich it 

 passes out again, is a single ganglion, which sends fila- 

 ments to both of them, and other branches which spread over 

 the surface of the mantle. "NATien any substance is dra-wn 

 in by the current, the entrance of which would be injiu-ious, 

 it excites a general contraction of the mantle, and this causes a 

 jet of water to issue from both orifices, which carries the offend- 

 ing body to a distance, and in the same manner, if the exterior 

 of the body be touched, the mantle suddenly and violently con- 

 tracts. These are the ouly actions which, so far as is known, the 

 nervous system of these animals is destined to perform. They 

 do not exhibit the least traces of eyes or other organs of special 

 sense ; and the only parts that appear peculiarlj' sensitive are the 

 small teutacula wliich guard the orifice by which the water 

 enters. It would seem as if the irritation caused by the con- 

 tact of any hard substance with these, or with the general surface 

 of the animal, caused an instinctive contraction of the mantle, 

 having for its object to get rid of the source of the irritation. 

 Such a movement could only be performed hy the aid of a 

 nervous system, which has the power of receiving impressions 

 and of immediately exciting even the most distant parts of the 

 body to act in accordance with them. In the SensitiA^e plant and 

 Yeuus' fly-traji an irritation applied to one part is the occasion 

 of a movement in another ; but this takes place slowly, and in a 

 manner very different from the energetic and immediate con- 

 traction of the mantle of the Tunicata. The larva of the com- 

 pound Sessile Ascidians is like the tadpole of a frog, which 

 swims about for a time ; it then fixes itself by the head to some 

 object, the tail falls off, and in a few days it becomes a solitary 

 Ascidian, with its two orifices and currents of water. This soli- 

 tary animal gives origin by budding to a connected group, which 

 in its turn lays fertilized eggs, so that there is an alternation of 

 generations. These Tunicata live on diatoms and morsels of sea 

 weed, and, like all fixed Ascidians, they show no external sign 

 of vitality except that of opening and shutting the two orifices. 



