i6 INTRODUCTION 



tunicates or "sea squirts" were later called mollusca. They are 

 characterized by a cellulose tunic, retrogressive metamorphosis 

 and reversible heart beat. 



Invertebrates 



Having described briefly the Urochorda and Adelochorda, which 

 we may consider types intermediate between the Vertebrates and 

 the Invertebrates, let us consider the characteristics that differen- 

 tiate the Invertebrate Phyla. We have seen that all the Verte- 

 brates belong to the same Phylum, and are to a great extent related, 

 but the Invertebrate Phyla are widely different in characteristics. 



1. Invertebrates have neither internal skeleton nor notochord. 



2. For the most part their nervous system is ventral to the di- 

 gestive tract. 



3. They lack gill slits or visceral clefts. 



4. When present, the heart is dorsal. 



Phylum Arthropoda (450,000 species). — Segmented animals, 

 some with jointed appendages. Body covered by a chitinous exo- 

 skeleton secreted by cells beneath It; bilaterally symmetrical, with 

 anus but poorly developed coelom. (Examples — crab, spider, mos- 

 quito.) 



Phylum Mollusca (60,000 species). — Unsegmented with no true 

 appendages. Fundamental bilateral symmetry Is lost in Gastro- 

 poda; while the ventral muscular foot characteristic of the group is 

 subject to modification. Frequently a bilobed shell is present with 

 the mantle, a dorsal fold of the body wall covering the animal. 

 Sometimes both shell and mantle are absent; but a coelom and anus 

 are present. (Examples — oyster, clam, snail and octopus.) 



Phylum Molluscoidea (2,000 species). — a. Bryozoa or Polyzoa- 

 Colonlal, like some Coelenterates. Complete alimentary canal. 

 Large body cavity. (Pectinatella is the commonest fresh water 

 form.) b. Brachiopoda — once gigantic bivalves, rulers of the ocean. 



Phylum Trochelminthes (500 species). — Somewhat resembling 

 the Infusorian protozoa, the Rotifers are often called "wheel ani- 

 malcules." Well developed digestive system with mouth, mastax 

 (chewing stomach), glandular stomach, intestine and anus. Fe- 

 males large, males few In number and small; resemble larvae of 

 annelids and molluscs. 



Phylum Annelida (4,000 species). — Visibly segmented worms 

 with three cellular layers. (Triploblastic.) No jointed appen- 



