314 Animal Biology 



tuskshaped, calcareous shell is secreted by the mantle. They possess bilateral sym- 

 metry and the head is rudimentary. 



Example: Tooth shell (Dentalium) (Fig. 125). 



Phylum 11. Arthropoda (ar -throp'o da) (Gr. arthron, jointed; pous, appendage, 

 or foot) 



General Characteristics 



Paired, jointed appendages are present on all or some of the segments of the 

 body. An exoskeleton of chitin is secreted by the cells just beneath it. Chitin 

 (Gr. chiton, a tunic or covering) is a protein material and has the formula 

 (C50H30O19N4) . The external, dissimilar segments (metameres) are well defined, 

 but the internal segments are largely obliterated. The body possesses regional 

 specialization (certain regions for specific purposes) and is bilaterally symmetrical. 



\ Vh(?j; forbonnq 



Ihcurrenb siphon-^^^S^-^ipnon 



Fig. 122. — The shipworm (Teredo navalis) of the class Pelecypoda, phylum 

 Mollusca. The shipworm is shown somewhat diagrammatically in its burrow in 

 a piece of wood. The modified bivalve shell and the siphons are characteristics 

 of mollusca. 



The animals are triploblastic, having three germ layers: ectoderm, mesoderm, 

 and entoderm. The coelom (body cavity) is rather poorly developed, it being 

 replaced by a hemocoel (Gr. hema, blood; koilos, cavity) filled with blood. The 

 mouth and anus are on opposite ends of the animal. A tubular heart and its 

 aorta are dorsal to the alimentary canal. Blood sinuses are commonly distributed 

 throughout the tissues. The nerve cord with its ganglia is ventral to the alimen- 

 tary canal. The paired cerebral ganglia are anterior and dorsal to the alimentary 

 canal (as in the earthworm). The cerebral ganglia are connected with the ven- 

 tral nerve cord by a nerve ring. The muscles of the body are usually striated 

 (striped). Number of species of Arthropoda, 675,050. 



Classification of Arthropoda (see table on pp. 318 and 319) 



