APPENDICULARIA. 



459 



It undergoes fatty degeneration, and is cast off together 

 with the entire notochord. The tail-less body becomes a 

 shapeless bag, or sac, which, by retrograde metamorphosis 

 of its separate parts and by re-formation and modification, 

 gradually acquires that remarkable structure which has 

 already been described. 



Fig. 162. — Appendicularia (Copelata), 

 seen from the left side : m, mouth; k, gill- 

 intestine ; o, oesophagus ; v, stomach ; a, 

 anus ; n, brain (upper throat ganglion) ; 

 g, ear-vesicle ; /, groove under the gill ; 

 h, heart ; t, testes ; e, ovary ; c, notochord ; 

 s, tail. 



Among the extant Mantle 

 Animals (Tunicata) there is, how- 

 ever, an interesting group of 

 small animals which retain 

 throughout life the tailed, inde- 

 pendent ascidian larval stage of 

 development, and which, by 

 means of their permanent, broad, 

 rudder-like tails, move actively 

 about in the sea. These are the 

 remarkable Appendi cidarice (Fig. 

 162). They are the only extant 

 Invertebrates permanently pos- 

 sessing a notochord, and are, 

 therefore, the nearest allies of 

 the extinct Chorda Animals 

 (Chordoma), of the primaeval 



Worms which must be regarded as the common parent-form 

 32 



