ANIMAL METAMORPHOSIS. 95 



We next come to the Ttmicata — the Ascidians, or sea-squirts — 

 many of which undergo a very definite metamorphosis. These 

 marine animals, which have been compared to a double-necked 

 leathern bottle, are found immoveably attached to rocks, stones, 

 and weeds on the sea-bottom (Fig. 12). They are protected by a 

 tough elastic integument, with two openings, situated close toge- 

 ther at the free extremity of the animal, through which a constant 

 current of water passes. The egg of the Ascidian gives rise to an 

 active, free-swimming organism shaped very much like the tadpole 

 of a frog, which it resembles not only in general form, but also in 

 its internal structure (Fig. 13), the only important difference being 

 in the position of the two mouths, and in the fact that the Asci- 

 dian has one eye, while the frog has two. The axis of the tail 

 consists of a cartilaginous or gristly rod (Fig. 13^), thus fore- 

 shadowing the notochord or primitive backbone of vertebrate 

 animals, on which account it has been proposed by some zoolo- 

 gists to raise the Ascidians to the rank of a sub-kingdom. Button- 

 like warts bud out from the fore-part of the head, by which, when 

 the time for its transformation arrives, the infant Ttunicate cements 

 itself to some fixed body. The tail atrophies, the body grows, 

 and gradually changes, its shape becoming wide and sac-like, a 

 large cloacal chamber forms, the eye disappears, the outer skin 

 becomes tough and leathery, and from being a free-swimming, 

 active tadpole, it becomes a mere stationary sac, absorbing 

 nourishment and laying eggs (Fig. 12). 



EXPLANATION OF PLATE XL 



Fig. 1. — Larva of Echinus (after J. MuUer). A, Anus ; F, Mouth 

 processes ; B, Posterior side-arm ; a, Mouth ; a', (Esophagus ; 

 b, Stomach; 6', Intestine; d, Ciliated bands ; f.f.f., Ciliated 

 epaulettes; c, Disc of future Echinus. 



,, 2. — Larva of Star-fish, Bipinnaria asterigera. a, Mouth ; a', 

 (Esophagus ; h, Intestinal tube and Anal orifice ; c, Furrow, 

 in which the mouth is situated ; d,d', Bilobed peduncle ; 

 1, 2, 3, 4, 5,. (), 7, Ciliated arms. 



,, 3. — Larva or Pluteus form of the Brittle-star, Ophiolepis. m., 

 Mouth ; s. , Stomach ; c. , Calcareous skeleton. 



J J 4. — Larva, with rudiments of the stem- joints of Comatula. 



