622 EVOLUTION OF ANIMALS Part V 



A Review of Arthropod Relations 



Again and again arthropods show their ancestral connections to annelid 

 worms. Peripatus (Class Onychophora — "claw bearing") is the simplest living 

 arthropod and with its segmentally arranged excretory organs and wormlike 

 form most resembles the annelids. Centipedes (Class Chilopoda — lipfoot) and 

 millipedes (Class Diplopoda — doubled feet) have mainly uniform segments. 

 The voracious predatory centipedes are equipped with powerful mandibles 

 each with an incurved hook from the tip of which a poison gland opens. In 

 contrast to them the vegetarian millipedes have weak mandibles and no poison 

 glands. A centipede is composed of flattened segments, each with one pair of 

 long, jointed legs; a millipede is cylindrical and each segment is a fusion of 

 two embryonic ones bearing two pairs of legs. When traveling these various 

 legs are moved from front to rear rapidly like scales being played on a key- 

 board. Crayfishes (Class Crustacea) are divided into a fused head and thorax, 

 and abdomen and have gills, two pairs of antennae, and two-branched appen- 

 dages. In grasshoppers (Class Insecta), the body is divided into head, thorax, 

 and abdomen, and they have three pairs of legs, one pair of antennae and are 

 usually winged. The bodies of spiders and their allies (Class Arachnoidea) are 

 divided into a cephalothorax and abdomen; they are without antennae or 

 mandibles, have four pairs of legs, and breathe by tracheae and book lungs. 



Spiders and Tlieir Relatives 



Spiders are well named for the majority of the females are inveterate spin- 

 ners and the word spider is a descendant of the Danish word spinden, to spin. 

 For most spiders silk is the thread of life from the time they hatch from the 

 shell. Spiders are air breathers, thoroughly land animals, yet inside of silken 

 waterproofs a few of them live in water. Some occupy silk curtained holes in 

 coral rocks that are immersed at high tide. The "water spider" (Argyroneta) 

 of fresh waters of Europe and temperate Eurasia is a pioneer user of the diving 

 bell. She collects her supplies of oxygen at the water surface raising her ab- 

 domen and capturing bubbles of air in addition to that caught on the covering 

 of her body. Between repeated trips to the surface she weaves a canopy of silk 

 attaching it to the submerged stems of plants that grow in the shallows of ponds 

 and streams. After the canopy is made she continues to bring down air bubbles 

 and to shed them beneath the canopy replenishing the supply as it is used. 

 This airy chamber is the home of the female spider into which she brings her 

 captured prey, and where she lays her eggs. The spiderlings that hatch there 

 can also spin and swim and with their own silk soon repeat the performances 

 of their mother. The males spin only small canopies sufficient for them to 

 linger in the locality until they are mature. There are many spiders that fre- 

 quent the margins of quiet inland waters, running about on the surface film 



