298 TADPOLES OF THE SEA 



of colours contrasting with that of the sheet-like 

 tunic in which they are embedded, so that the sheet- 

 like growth looks like a piece of woven fabric with 

 a radiated star pattern repeated all over its surface. 

 Purple and green, yellow and orange in various shades 

 are the colours contrasted in the stars and groundwork. 

 This extraordinary arrangement of little animals as 

 flattened sacs grouped like the rays of a star in a gela- 

 tinous bed or matrix is as unlike a vertebrate fish or tad- 

 pole as anything can be ; yet the animals composing this 

 star-spangled jewel actually lay eggs, from each of which 

 a tadpole hatches ! The Botryllus, as this beautiful 

 encrustation is called, is one of those exceptional Ascidians 

 which have retained the tadpole phase, or young form (often 

 called a " larva," the name applied to the caterpillar of 

 moths and butterflies), in their growth from the egg. The 

 Botryllus tadpole, after some days of a free-swimming 

 life, fixes itself by its head to a solid stone or weed, and 

 changes into a star-shaped group, from which other con- 

 nected star-groups are budded off, whilst remaining 

 embedded in a common gelatinous sheet exuded by 

 them, and holding them together ! 



Though we can often prove that the brilliant colouring 

 and pattern-marks of an animal or a plant are of advan- 

 tage to their possessor, either as attracting other animals 

 to it or as concealing it from enemies or warning others 

 to leave it alone, yet there is a vast preponderance of 

 cases in which we cannot ascribe any special " use " to 

 the colour, as colour. It happens that certain chemical 

 bodies necessarily manufactured by the plant or animal in 

 the course of its living changes are of this or that colour, 



just as water is blue and iron-rust is red, and so there 



the brilliant colour is. And so far as the advantage of 

 the animal or plant is concerned, it might just as well not 

 be there at all ! This uselessness of beautiful colouring 



