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THE POPULAR SCIENCE MONTHLY. 



which mark the phases of ascidian life-history. Thus it is matter of 

 sober, natural-history fact that a sea-squirt larva, of all invertebrate 

 animals, is the only being that possesses organs and parts proper to 

 the young vertebrate or to the adult form of one lower vertebrate in 



Fig. 16. Sea-Squirt. 



Fig. 17. Development of Sea-Squirt. 



particular. This adult is the little fish known as the lancelet, which, 

 in the relative simplicity of its organization, makes a nearer approach 

 to the poor or sea-squirt relations of the vertebrates than any other 

 fish. 



The fact of vertebrate and sea-squirt relationship is worth dwell- 

 ing upon, because the topic unquestionably presents one with a com- 

 mon point of view, whence a survey of the higher development, evolu- 

 tion, and progress of the vertebrates, and a view of the degeneracy 

 and retrogression of the sea-squirts, may best be obtained. Reveling 

 in the freedom of its early life, the larval sea-squirt presenting, as 

 already noted, a striking resemblance to the tadpole of the frog, in its 

 backbone, its nerve-system, and its breathing-sac, or modified throat 

 ultimately settles down. Like the youthful barnacle somewhat, the 

 young sea-squirt attaches itself to a stone or shell by the suckers with 

 which nature has provided its head. Then succeeds the disappearance 

 of the tail, with its backbone and its nerve-cord, and the body itself 

 soon assumes the sac-like shape that betokens the mature ascidian char- 

 acter. The outer skin becomes tough and leathery, and develops the 

 cellulose which, by biological right, we should expect to find in plants 

 alone. Then succeeds the fuller formation of the gill-sac or breathing- 

 chamber, and of its neighbor compartment, which receives the effete 

 water of respiration to be ejected by the second mouth of the sac-like 



