THE SEA-SQUIRT'S YOUNG 283 



apparent. The Ascidians, like other animals, produce 

 eggs, which are duly fertilised, and in many cases grow 

 without any delay into the shape and form of their 

 parents. But fortunately for our information in regard 

 to a very interesting chapter of natural history not all 

 the kinds of Ascidians grow as directly and simply as pos- 

 sible from the egg to the parental form. Some (and our 

 common Ascidia mentula is among them) actually hatch 

 from the egg as tadpoles smaller than the full-sized 

 tadpoles of the common frog, but nevertheless tadpoles 

 tadpoles with oval, conjoined head and body, as in those 



ASC1D1AN 



FIG. 34. The tadpole of a frog and of an Ascidian compared both 

 much enlarged, the Ascidian more than the frog. A very notice- 

 able difference is that the mouth of the Ascidian tadpole is on the 

 top of its head, whilst that of the frog's tadpole is in front and faces 

 downwards. 



familiar to us, and a long, fin-fringed tail, which, by its 

 rapid, wriggling strokes to right and left, drives the little 

 creature through the salt water (Fig. 34). 



These are our " tadpoles of the sea," Ascidian tadpoles. 

 Their superficial appearance was known to naturalists 

 for many years before the details of their internal structure 

 and its mode of formation from the first embryonic cells 

 into which the egg-cell divides were ascertained. The 

 resemblance of the little marine Ascidian tadpoles, a quarter 

 of an inch long, to a frog's tadpole was noted, and regarded 



