PHYLUM XI, ARTHROPODA 



The Arthropoda are transversely segmented animals with 

 mouth and anus at opposite ends of an elongate body, with 

 muscles attached to the inside of an external skeleton (the very 

 opposite of the vertebrates), and with a nervous system composed 

 of a dorsal brain above the oesophagus, connected by nerves 

 around this with a double ventral chain of segmentally arranged 

 ganglia. A few or most of the body segments bear paired append- 

 ages, the distinct segments of which latter are separated by 

 joints and moved by special muscles. Thus, as the animal does 

 not need the movement of the skin for locomotion (as in the 

 annulate worms) the skin secretes a horn-like substance, chitin, 

 to form a hard outside skeleton. Since this rigid exoskeleton 

 prevents increase in size, growth can occur only through the 

 shedding or molting of this chitin. Eyes are nearly always 

 present and are simple or compound. The cavity of the body 

 between the exoskeleton and the internal organs consists largely 

 of spaces, the blood sinuses, which are in full communication 

 with the blood circulatory system. The heart, usually present, 

 is elongate and is situated in a blood sinus from which the blood 

 enters by valvular openings (ostia) . 



Arthropods are probably descended from some annelid worm- 

 like ancestor, the crustaceans continuing to live in the water, 

 while the insects, myriopods and most of the arachnids are di- 

 verging to a life upon the land. 



Derivation of name. — Greek arthron, ]o\\\t, + pous (pod), 

 foot, referring to the division of the limbs into movable segments. 



The phylum is subdivided as follows : — 

 Sub-phylum i, Branchiata. — Mostly water breathers; breathe 

 by gills (branchiae). 



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