780 



CLASS V. ARACHNIDA. 



The reproductive organs with the ducts frequently form a 

 ring, sometimes a network, which encircles the alimentary canal 



and nervous system. 

 The oviducts and vasa 

 deferentia unite to 

 open by a single, 

 median, ventral pore, 

 except in Limulus, 

 which has paired 

 pores. The ring-like 

 ovary and testis of 

 the Acarines and 

 Phalangids have be- 

 come in the scorpions 

 and in Limulus reti- 

 f orm and lie embedded 

 among the digestive 



FIG. 507. Male and female generative organs of Phalan- 

 gium opilio (after Krohn). T Testis ; Vd vasa defer- 

 entia ; P penis with accessory glands ; R retractor 

 muscles ; Ov ovary ; U uterus ; Op ovipositor. 



glands. Receptacula 



seminis and vesiculae seminales are usually swellings on the ducts, 

 but in spiders the former are distinct from the oviducts and 

 open just in front of the female opening. Accessory glands are 

 also usually present but their exact function is still obscure. 



The Pedipalps and scorpions and some mites are viviparous, 

 the ova developing in the ovary or uterus. Many spiders and 

 Pseudoscorpions attach their eggs to their bodies. As a rule 

 the young are miniatures of their parents, but mites are often 

 hatched with but three pairs of walking limbs and pass through 

 a series of stages, separated by a moult, before becoming adult. 

 The Phalangids die down during the winter and the race is then 

 represented only by developing eggs. 



Certain Acarines are parasitic on plants and on animals, and 

 some of them as well as some spiders are aquatic, but the great 

 majority of Arachnids are terrestrial. Limulus is marine. Many 

 are nocturnal such as the Solifugae and Phalangids, and many, 

 such as the spiders, weave w r ebs and have specialized " silk " 

 glands. In some cases, markedly in the Acarines, enormous 

 numbers of individuals live together, but they show no adaptations 

 to a communal life and no individuals, specialized to perform cer- 

 tain pieces of work, occur as they do occur among the social 

 insects. Of the air-breathing Arachnids the earliest fossils. 



