146 -NATURE STUDIES. 



it is discharged into tlio outer world. The sea 

 squirt's stomach. (B) opens from the throat, and its 

 intestine in turn opens into the atrium. A heart (?) 

 exists in the shape of a curious tube, which propels 

 the blood for so many beats in one direction, and 

 then, reversing its action, sends that fluid for so 

 many pulsations in the opposite direction. A single 

 nervous mass lies between the two openings of the 

 body, and the other wall of the body itself consists 

 largely of a material called cellulose, which is a com- 

 mon plant compound. The animal in such a case 

 manufactures the substance of the plant, and imitates 

 the chemistry of the latter organism. 



Such is an outline of the rooted and fixed <e sea 

 squirt." Its earlier history, however, is still more 

 interesting. It developes from the egg in exactly the 

 same fashion as the lancelet. The early stages of the 

 sea squirt run parallel with those of that fish. Finally, 

 at a certain stage, the young sea squirt makes its 

 appearance as a free-swimming active body, somewhat 

 resembling a tadpole, and possessing a long tail. 

 More wonderful still, the resemblance to the lancelet 

 has become of a closer character. A notoclwrd is 

 developed, as in the lancelet and young of other Ver- 

 tebrata, and a nervous system, appears above the 

 notoclwrd, as in the lowest fish. So, also, the charac- 

 teristic throat, with its clefts, is formed. Now, whilst 

 certain sea squirts appear to remain in this tadpole- 

 staire, most of the species begin sooner or later to 

 retrogress and to backslide in development. The 



