4 TEXT-BOOK OF ENTOMOLOGY 



We will now rapidly review the leading features of the classes of 

 Arthropoda. 



The Crustacea. These Arthropoda are in many most important 

 characteristics unlike the insects; they have two pairs of antennae, 

 five pairs of buccal appendages, and they are branchiate Arthropoda. 

 They have evidently originated entirely independently, and by a 

 direct line of descent from some unknown annelid ancestor which 

 was either a many-segmented worm, with parapodia, or the two 

 groups together with the Rotifera may have originated from a com- 

 mon appendigerous Trochosphsera. Their segments in the higher 

 forms are definite in number (23 or 24) and arranged into two 

 regions, a head-thorax (cephalothorax) and hind-body (abdomen). 

 Nearly all the segments, both of the cephalothorax and abdomen, 

 bear a pair of jointed limbs, and to them at their base are, in the 

 higher forms, appended the gills (branchiae). The limbs are in the 

 more specialized forms (shrimps and crabs) differentiated into eye- 

 stalks, two pairs of antennae, a pair of palpus-bearing jaws (mandi- 

 bles), two pairs of maxillae and three pairs of maxillipeds; these 

 appendages being biramose, and the latter bearing gills attached to 

 their basal joints. The legs are further differentiated into ambula- 

 tory thoracic legs and into swimming or abdominal legs, and in the 

 latter the first pair of the male is modified into copulatory organs 

 (gonopoda). The male and female reproductive organs as a rule are 

 in separate individuals, hermaphrodites being very unusual, and 

 the glands may be paired or single. The sexual outlets are gener- 

 ally paired, and, as in the male lobster and other Macrura, open in 

 the basal joint of the last pair of legs,- and in the female in the 

 third from the last; while originally in all Crustacea the sexual 

 organs were most probably paired (Fig. 3, B). 



They are, except a few land Isopoda, aquatic, mostly marine, and 

 when they have a metamorphosis, pass through a six-legged larval 

 stage, called the Nauplius, the shrimps and crabs passing through an 

 additional stage, the Zoe'a. Crustacea also differ much from insects 

 in the highly modified nature of the nephridia, which are usually 

 represented by the green gland of the lobster, or the shell-glands of 

 the Phyllopoda, which open out in one of the head-segments ; also in 

 the possession of a pair of large digestive glands, the so-called liver. 



Intermediate in some respects between the Crustacea and insects, 

 but more primitive, in respect to what are perhaps the most weighty 

 characters, than the Crustacea, are the Trilobita, the Merostomata 

 (Limulus), and, finally, the Arachnida, these being allied groups. In 

 the Trilobita and Merostomata (Limulus), the head-appendages are 



