530 THE INVERTEBRATA 



Class ARANEIDA 



Arachnids with prosoma covered by a single tergal shield but head 

 marked off by groove ; opisthoSoma ("abdomen ") separated by waist, 

 soft, rarely having any trace of segmentation, two to four pairs of 

 spinnerets and several kinds of spinning glands ; chelicerae two-jointed, 

 subchelate; pedipalps modified in male for transmission of sperm. 



In the embryo spider, the segmentation of the opisthosoma is in- 

 dicated by the presence of coelomic cavities of which there are ten 

 (Fig. 352 B) ; there are also five pairs of rudimentary appendages, the 

 first of these disappears, the next two assist in forming the lung books, 

 and the fourth and fifth become the spinnerets. When more than two 

 pairs of spinnerets are present the additional ones are split off from 

 pre-existing spinnerets. Embryology thus shows that the existing 

 forms with apparently unsegmented opisthosoma are descended from 

 ancestors with nearly the full number of segments typical of arachnids. 



The chelicerae (Fig. 364) contain a poison gland in the basal joint. 

 Spiders have developed to an extreme the tendency, so common in the 

 arachnids, towards adopting a carnivorous diet. While most of the 

 spiders on account of their size can only obtain suitable supplies of 

 food from insect life, some are able to attack larger forms, even birds 

 in the case of Mygale. Besides the poison glands which cause the 

 immediate death of the prey, there are salivary glands in the under lip 

 which produce a proteolytic ferment. A fly which is caught by a 

 spider is pressed against the mouth by the gnathobases of the pedi- 

 palps, a drop exudes from time to time and in a couple of hours the 

 morsel of flesh has been externally digested and the resulting fluid 

 sucked into the spider's alimentary canal by the pulsations of the 

 "stomach", the chitinous exoskeleton of the prey remaining as an 

 empty husk. This method of feeding is a leading characteristic of the 

 group. 



The diagram (Fig. 361) shows the main features of the anatomy 

 of the spider. The oesophagus, after dilating into the sucking stomach, 

 is succeeded by the mid gut which immediately sends out two main 

 lateral branches forward with coeca running into the limbs. It passes 

 back through the opisthosoma and gives place to the hind gut where 

 the Malpighian tubules are given off. The main feature is the digestive 

 gland which is a dorsal diverticulum of the mid gut, richly branched 

 and filling the opisthosoma on each side of the heart. In this the latter 

 stages of digestion take place. The hind gut is short and dilated into 

 a stercoral pocket where faeces accumulate. The heart is situated in a 

 distinct pericardium in the opisthosoma, has three pairs of ostia, and 

 gives off an anterior and a posterior aorta and three lateral arteries on 

 each side. In contrast to the scorpion and Limulus there are no definite 



