CHAP. XVI Backboned Animals 251 



bodies, while the title Tunicate refers to a. characteristic cloak ot 

 tunic which envelops the whole animal. 



There is not much to suggest backbonedness about these Tuni- 

 cates, and till 1866 no one dreamt that they could be included in 

 the Vertebrate series. But then the Russian naturalist Kowalevsky 

 discovered their life-history. The young forms are free-swimming 

 creatures like miniature tadpoles, with a dorsal nerve-cord, a sup- 

 porting rod in the tail region, gill-slits opening from the food canal, 

 a little eye arising as an outgrowth of the brain, and a ventral 

 heart. 



There are only two or three genera of Tunicates, especially one 

 called Appendicularia, in which these Vertebrate characteristics are 

 retained throughout life. The others lose them more or -less com- 

 pletely. The young Tunicates are active, peifhaps too active, for a 

 short time ; then they settle down as if fatigued, fix themselves by 

 their heads, absorb their tails, and become deformed. The nervous 

 system is reduced to a single ganglion between the two apertures ; 

 the original gill-slits are replaced by a great number of a different 

 character ; the eye is lost. From the skin of the degenerate animal 

 the external tunic is exuded. It is a. cuticle, and consists, in part 

 at least, of cellulose, the substance which forms the cell-walls of 

 plants. Thus this characteristically vegetable substance occurs 

 almost uniquely in the most passive part of a very passive animal. 

 The sea-squirt's metamorphosis is one of the most signal instances 

 of degeneration ; the larva has a higher structure than the adult ; 

 the young Tunicate is a Vertebrate, the adult is a nondescript. We 

 cannot tell how this fate has befallen the majority, nor why a few 

 are free-swimmers, nor why Appendicularia retains throughout life 

 the Vertebrate characteristics of its youth. Do the majority over- 

 exert themselves when they are " tadpoles," or are they constitu- 

 tionally doomed to become sedentary ? 



Tunicates are hermaphrodite — a very rare condition among Ver- 

 tebrates ; some of them exhibit " alternation of generations," as the 

 poet Chamisso first observed ; asexual multiplication by budding is 

 very common, and not only clusters but more or less intimate 

 colonies are thus formed. 



Tunicates live in all seas, mostly near the coast from low water 

 to 20 fathoms, and usually fixed to stones and rocks, shells and sea- 

 weed. A few are free-swimming, such as the fire-flame (Pyrosoma), 

 a unified colony ef tubular form, sometimes 2 or 3 feet in length, 

 and brilliantly phosphorescent. Very beautiful are the swimming 

 chains of the genus Salpa, whose structure and life-history alike are 

 Complicated. 



Tunicates feed on the animalcules borne in by the water 

 currents, and some of them must feed well, so rapidly do they grow 



