410 



AETIIROPODA. 



the organs, and is circulated in an irregular manner by the move- 

 ments of the different parts of the body. Not unfrequently (Achtheres 

 and Cyclops) the circulation is effected by the regularly repeated 

 movements of certain organs (intestine, vibratile plates, etc.) ; in 

 other cases, a short saccular heart is present dorsally above the 

 intestine ; or a long vascular tube (the dorsal vessel), divided into 

 chambers, serves as a propelling organ. From this, vessels (arteries) 

 _^ may arise, which conduct the blood in 



definite directions. Vessels for returning 

 the blood (veins) may also be present. 

 These either begin in the body cavity, 

 or are connected with the ends of the 

 arteries by capillary vessels. The vascular 

 system seems never to be completely 

 closed, since even when the circulation is 

 most complete, lacunar spaces of the body 

 cavity are found inserted in the course 

 of the vessels. 



Respiration is very frequently effected, 

 especially in the smaller and more deli- 

 cate species of Arthropoda, by means of 

 the entire surface of the body. In the 

 larger aquatic forms, the function of respi- 

 ration is assumed by special tubular, 

 usually branched appendages of the limbs 

 (branchiae) ; while in the air-breathing 

 Insects, Centipedes, /Scorpions, and Spiders, 

 respiration is performed by means of in- 

 ternal branched tubes filled with air 

 (trachece) or by pulmonary sacs (fan 

 tracheae). 



The reproduction of the Arthroy>oda is 

 usually sexual, but sometimes takes place 

 by the development of unfertilized ova 

 (parthenogenesis). Ovaries and testes are in 

 their origin paired, as are also the gene- 

 rative ducts, which often have a common terminal portion and 

 open by a median generative aperture (Insecta, Arachnoidea}. With 

 a few exceptions (Cirripedia, Tardigrada), the sexes are separate. 

 Males and females frequently differ essentially in their entire form 

 and organization. In rare cases, for example in the parasitic 



FIG. 329. Alimentary canal of 

 Pontia brassier (after New- 

 port). 2t, Proboscis (Maxillae) ; 

 Sp, salivary glands ; Oe, oeso- 

 phagus ; 8, sucking stomach ; 

 My, ilalpighian tubes ; Ad, 

 rectum. 



