GENERAL SURVEY OF THE ANIMAL KINGDOM. s 



Annelid worms, but clad in firm armature of chitin and 

 possessed of jointed appendages to which the name refers. 



Crustacea. Crabs, crayfish, "water-fleas," etc. 



Protracheata. Peripatus, probably a connecting - link between 

 some worm-type and Insects. 



Myriopoda. Centipedes and millipedes. 



Insecta. 



Arachnoidea. A miscellaneous class, including not only spiders 

 and scorpions, but such divergent forms as mites, and pro- 

 bably the king-crab (Limulus), and the extinct Eurypterids 

 and Trilobites. 



Another branch of the genealogical tree, well-defined 

 from the others, bears the shelled animals or Mollusca. 

 They are unsegmented, and without appendages, and pro- 

 bably derived from some worm type. 



1. Lamellibranchiata. Bivalves. 



^. Gasteropoda. Snails. 



3. Cephalopoda. Cuttle-fishes. 



And one or two smaller classes. 



VERTEBRATE METAZOA. 



Near the starting-point of this higher series, we must 

 place two remarkable worm-like animals — Balanoglossus and 

 Cephalodiscus. They have a dorsal nerve- cord as well as a 

 ventral ; in the anterior region of the body there is a slight 

 supporting rod towards the dorsal surface ; there are re- 

 spiratory slits opening from the beginning of the food 

 canal to the exterior. In short, the Vertebrate afifinities are 

 well marked, and we shall at least emphasise the fact that 

 there are no hard and fast lines of division if we place these 

 two types at the beginning of the Chordate series. They 

 form the class Enteropneusta (gut-breathers), or Hemi- 

 CHORDATA (half-chordate). 



Stumbling at the threshold, as it were, are the sea-squirts 

 or TuNicATA, or Urochordata, which are distinctly Chor- 

 date when they are young, but usually degenerate in adol- 

 escence. The young have a dorsal nerve-cord, a dorsal axis, 

 gill-slits, a ventral heart, a " brain-eye," but with the excep- 

 tion of a few like Appendicularia, which remain always 

 young, the adults lose or disguise these Chordate characters. 



