Animal Life by the Sea-shore. 



II.— ASCIDIANS. 



On inspecting forms of life in rock pools and oyster- 

 beds the observer's eye cannot fail to be attracted by berry-like 

 red or pink creatures fixed to stones, oysters or fucus-fronds, 

 provided with two funnel-shaped openings, through which, when 

 touched, jets of water are ejected with considerable force, 

 hence the name sea-squirts popularly'' bestowed on these 

 creatures, scientifically known as Ascidians or Tunicates. Look- 

 ing at one of these sea-squirts, the Currant Squirter [Cynihta 

 grossularia), for instance, one of the commonest species on our 

 coasts, the uninitiated would surely take it to be a very low 

 form of the animal kingdom, yet, as we shall explain, it is placed 

 in the zoological sj'stem close to the Vertebrates or backboned 

 animals, and forms part of a great division which includes the 

 Lancelet, long regarded as a fish, and mentioned, in connexion 

 with fishes, at the end of the preceding chapter. This proposition 

 may appear almost incredible when we consider the perfect 

 creature alone ; its justification is afforded by a knowledge 

 of the much more highly organised larval form, free-swimming 

 and tadpole-like, which will be briefl}' described. Previous to 

 doing so, however, we must impress upon the mind of the 

 beginner in zoologj' that, in certain tj'pes of animals, the 

 perfect or adult condition constitutes a state of degradation, 

 a simplification of the organism, which obscures its natural 

 relationships. Further on we shall have occasion to adduce 

 another perfectly similar instance in the case of the Barnacles 

 and the Sacculine, the position of which among Crustaceans 

 did not become apparent until a knowledge of their transforma- 

 tions during development had been acquired. The theory of 

 evolution does not necessarily imply a gradual ascensional 

 series ; far from it, it holds that many animals, instead of 

 competing with equally or better organised forms, have main- 

 tained themselves by acquiring a simplified structure \vhich 

 fits them for life under certain conditions, thus to fill vacant 

 positions in the economy of Nature ; hence we talk of them as 

 degraded or degenerate, taking into consideration their derix'a- 

 tion, as shown by earher stages in their development. 



