PHYLUM ARTHROPODA

Definition and Introduction

Bilateral  and protostomial eucoelomate  eumetazoa with metamerically  segmented  and each  segment bearing a pair  of  jointed  locomotory  appendages. Arthropoda  literally  means  animals bearing  jointed feet (Gr.  Arthron= jointed +podos=foot.) These  are extremely  successful animals  due to  a protective  exoskeleton   and jointed  appendages .That  is why more  than 8.1/2  lac  arthropod  species  are known  as against  about 2.1/2  lac species  of the  remaining  animals .

Brief History

Aristotle  described  a few carbs  and other  arthropods. Linnaeus  included all  such animals  in his  group insect Lamarck divided this  group into  three class  curstacea, Hexapoda and  arachnida  finally , von seibold (1845)  established the phylum arthropoda  for these animals.

Important Characteristics

1.       Occur widely on land, in  air  and in all  sorts of water,  from snowy tops of high mountains  to the  depths of ocean. Many  are parasite of other  animals  and plants . Hence  the phylum is  of great  economic  importance.

2.       Bilateral, triploblastic  body segmentd and also  regionated into head, thorax and abdomen. Segmentation marked  only externally  numbers  of segments or  somites fixed and  each  has its  separate exoskeleton  of thick and hard  chitinous    cuticle secreted  by epidermis of  body  wall. Head somites always  fused.

3.       Each  segment  basically  bears a pair  of lateral jointed  appendages adapted  for  food ingestion , locomotion , respiration , copulation,etc.

4.       Muscular system well developed , muscle fibres  always striated.

5.       Digestive  tract complete ,Most head appendages form mouth parts with  lateral jaws for  chewing  or sucking . Anus  terminal.

6.       Coelon reduced to  small cavities n in excretory  and reproductive organs  replaced elsewhere by blood  sinuses which  merge together  to form  a large  perivisceral cavity  the  haemocoel around  viscera, Sinuses  form  an open  blood  vascular system  filled  with haemolymph which  may  contain haemocyanin.  Haemocoel  communicates with a long  tubular and pulsatile  mid dorsal heart.

7.       Respiration  by gills (aquatic  forms )  or tracheae  or book  lungs (terrestrial forms ) by diffusion  through  body  surface in some.

8.       Excretion by coelomoducts or specialized  green or coxal  glands or ny  malpighian  tubules . 

9.       Nervous system basically similar  to the  typical  annelid plan head with  brain  ring  which  is connected  to a double  ventral  cord. Having  paired  segmental ganglia  which represent  true  metamerism .Well developed sensory   organs   or various types.

10.   Sexes mostly  separate  with sexual dimorphism .

11.   Fertilization typically  internal  in female body Eggs  megalecithal.  Oviparous or viviparous,

12.   Life cycle  includes one or  more  larval stages  that  metamorphose into adults.