488 TUNICATES 



ated or simple. It is a very intricate animal — a very complex 

 piece of vital machinery — which has specialised on a style of 

 its own, departing from that of its remote ancestors, to which 

 it returns for a brief space in its early youth. (4) One would 

 give much to understand the deep physiological significance of 

 the abandonment of the free-swimming habit. Do they over- 

 do it in their youth — these energetic tadpoles ; has the absence 

 of kidney-tubes anything to do with the cessation of energetic 

 activity; or is there some deeper vice in the constitution of 

 which the secretion of the cellulose test gives a possible hint ? 

 We do not know. It may, however, be at least suggested that 

 the whole history is the consequence of the sessile habit. Any 

 small aquatic animal may attach itself temporarily to solid ob- 

 jects. The Amphibian tadpole does so. If it is able to obtain 

 food without moving, it may remain attached longer, and if the 

 sessile habit is thus encouraged and rewarded, it may become 

 permanent. Thus the degeneration is the loss of structures 

 adapted to active life, and the acquisition of an organisation 

 adapted to stationary life. 



