PHYLUM ARTHROPODA 



323 



Malpighian tubule 

 Storage sa.c- 



Rectum 



Midgut - 



Anus 



Opanintfs to opisthosomal gastric poxichzs 



rHeart 



■ProsoTnad gastric pouch 



-Brain r'^oison gla.-na. 



<§-Ocdlus 



Cheliccra. 

 Fang 



Spiracle- 

 with trachaa 



SilH gland 



Ovar_y 



-Mouth 

 "Esophagus 

 -Subesophagealganglion 

 SucKin^ Stomach 



Figure 16.30. Internal anatomy of the orb-spider, Argiope. The body wall, ap- 

 pendages, and some of the internal organs of the right side have been removed. The much 

 branched gastric pouches in the opisthosoma are removed, leaving their openings into 

 the midgut. Only a few of the silk glands are shown. (After Buck and Keister.) 



on the tips of the chelicerae by way of slender ckicts. Prey is first killed 

 with poison from these glands, then wrapped tightly in silk from the 

 spinnerets. The spider applies its mouth to the prey and secretions con- 

 taining proteolytic en/ymes from glands behind the mouth begin diges- 

 tion. The resulting broth is sucked into the midgut where digestion is 

 completed and the nutrients are absorbed. 



The ovary lies in the opisthosoma beneath the midgut, opening 

 antero-ventrally by way of an oviduct through a genital pore between 

 the openings to the book lungs. Associated with the oviduct is a seminal 

 receptacle where sperm are received. The male is much smaller than the 

 female, Avith testes and a sperm duct in the opisthosoma. Before copula- 

 tion the male transfers the sperm to specialized cavities in the tips of his 

 pedipalps. At copulation the pedipalps are thrust into the female open- 

 ing and the sperm are expelled into the receptacle. The whole maneuver 

 is remarkably like that of the cephalopod molluscs. The eggs are fer- 

 tilized as they are laid and are put in a cocoon spun by the spinnerets. 

 They hatch later into miniature spiders. 



Associated with the three pairs of spinnerets are five kinds of silk 

 glands in the ventral part of the opisthosoma. The different secretions 

 yield different kinds of silk, including the nonsticky radial fibers of 

 the web, the sticky circular fibers, and the brownish fibers of the cocoon. 

 Silk is emitted as a fluid that instantly hardens into tough protein 

 threads. 



1 48. The Phylum Onycophora 



The Onycophora are about seventy species of wormlike, segmented, 

 terrestrial animals with metameric legs. All the living species, found in 

 very damp regions of the tropics, belong to one family, and possibly to 

 one genus, Peripatus (Fig. 16.31). 



During development, segmentation appears in Peripatus in a man- 



