ARTHROPODA 



6 9 



The circulatory organs consist of a contractile heart, a series of 

 arteries and afferent sinuses and veins. The excretory organs are 

 modified nephridia known as shell glands or green glands, located in 

 and opening from the anterior part of the body. The nervous system 

 consists of the brain, united by the circumoesophageal connectives with 

 'the ventral, ganglionated nerve cord. Sexes are separate or united; 

 parthenogenesis is common. 



The class is divided into two subclasses, the Entomostraca and the 

 Malacostraca. The former includes the smaller, simpler types, such as 

 water fleas, barnacles, etc., Figs. 49, 50, and 51; the latter includes the 

 larger, more highly organized forms such as crayfish, crabs, lobsters, 

 etc., some of which may be several feet in extreme length, Figs. 53 

 and 54. 



FIG. 50. Goose barnacle, Lepas sp. xM 



II. Onychophora, Fig. 52. This class includes the curious cater- 

 pillar, like genus, Peripatus, which is interesting as having character- 

 istics of both the Arthropods and the Annelids a sort of " connecting 

 type." 



III. Myriapoda, Figs. 56, 57 and 58. These are the centipedes and 

 millipedes. They have many of the characteristics of the insects. 

 There is a distinct head, bearing jointed antennae; a pair of eyes and two 

 or three pairs of jaws. The rest of the body is not marked off into 

 different regions, but is divided into many segments, which in some 

 species bear a single pair of appendages each (centipedes) and in other 

 cases two pairs of appendages each (millipedes). They breathe by a 

 system of trachea, like those of insects, opening to the exterior through 

 stigmata on the lateral or ventral side of the body. 



IV. Insects. These are the grasshoppers, bees, flies, beetles, butter- 

 flies, dragon-flies, etc. It is the largest of all the groups of animals, 



