SOME MOOT POIXTS IX NATURAL HISTORY. 241 



for its reception, and in modern manuals of zoology appears 

 as a far-off friend of the worms. 



No less remarkable in respect of their singular diver- 

 gence from their nearest allies, are those singular little beings 

 known as Appendicularians. These latter undoubtedly pos- 

 sess very close relations with the " sea-squirts," those rooted 

 sac-like animals found attached to rocks and stones on the 

 sea-beach at low-water mark ; but they differ from the 

 common sea-squirts in that they possess a tail and swim 

 freely about in the sea, presenting no small resemblance to 

 miniature tadpoles. Appendicularia also exists in the light 

 of a most singular being, since it illustrates what zoologists 

 call a " permanent larval form." All sea-squirts begin life 

 as tadpole-like beings (Fig. 35), but sooner or later lose 



FIG. 35. Young sea-squirt, or ascidian. 



their tails, and settle down into the fixed and rooted state 

 that characterises their wonted and usual existence. The 

 little appendicularia, however, retains its tail throughout life, 

 and in that it permanently retains its young features, illus- 

 trates as great an anomaly of sea-squirt existence as 

 would be constituted by the possession by humanity of the 

 frame of the child, associated with the age and belongings 

 of mature years. In this latter case also, naturalists have 

 had to construct a figurative dwelling-place for appendicu- 

 laria in the shape of a special division of the sea-squirt class, 

 and thus again the plasticity of living forms entails a specu- 

 lative hardship to the student of natural history. 



Occasionally it happens that a whole division of the animal 

 world, with its many included forms, may illustrate the case 

 of a house literally divided against itself. Such a division 

 is that containing as its tenants animals of such ill-assorted 



R 



