64 THE ARRAY OF LIVING ORGANISMS 



Exercises XII and Xlli 



THE ANIMAL KINGDOM 



Crustacea 

 (crabs, lobster) 



Insects 



Birds 



Mammals 



Arachnids (spiders, 

 horseshoe crab) 



Arthropods 

 ("jointed legs" 



Molluscs —^ 

 (snails, clams, 

 squid) 



Annelids — ^— 

 (segmented worms) 



Platyhelminthes- 

 (flatworms) 



- Reptiles 



- Amphibia 



-Bony fishes 

 -Cartilaginous fishes 



- Jawless fishes 



Vertebrates 



■ Roundworms, 

 rotifers 



h 



Chordctes 



Protochordates 

 (Amphioxus, acorn 

 worms, sea squirts) 



Echinoderms- 



PROTOSTOMIA 



DEUTEROSTOMIA 



Three cell layers 



Coelenterates 

 (corals, jellyfish) 



Porifera ■ 

 (sponges) 



Ctenophores 

 (comb jellies) 



Two cell layers 



PROTOZOA 



Of the approximately 1,200,000 species of liv- 

 ing animals, about 97% are invertebrates and 

 about 75% are insects. 



Guidelines 



(1) Two cell layers (sponges, coelenterates) to 

 three cell layers. 



(2) Development of a true body cavity 

 (coelom) lined with mesoderm. 



(3) Adaptations for emergence from water to 

 land and air. 



(4) Grouping of annelids, arthropods, and 

 molluscs as protostomes ("annelid super- 

 phylum") and of echinoderms and chor- 

 dates as deuterostomes ("echinoderm 

 superphylum"). 



The coelenterates and comb jellies are built 

 of two layers of cells, the internal endoderm 



lining the digestive cavity, and the external ecto- 

 derm. All further phyla add a third layer be- 

 tween these two, the mesoderm. 



The coelenterates and flatworms have only a 

 single opening into the digestive cavity, which 

 therefore serves as both mouth and anus. The 

 higher phyla possess tubular digestive systems, 

 open at both ends. In arriving at this condition, 

 the annelids, arthropods, and molluscs (proto- 

 stomes) convert the primitive opening to a 

 mouth, and break through a new opening for the 

 anus. In the echinoderms and chordates (deuter- 

 ostomes), the primitive single opening becomes 

 the anus, and a new opening is broken through 

 to form the mouth. 



One of the most important developments is 

 the formation of a body cavity (coelom) lined 

 with mesoderm, in which the internal organs lie. 

 The sponges and coelenterates have no meso- 

 derm, and no such cavity. The flatworms have 

 a solid mesoderm, and no cavity. The round 

 worms have a restricted mesoderm, and apart 



