324 ARACHNOIDEA AND PALMOSTRACA. 



cuticle of chitin, and also an interesting internal piece of 

 skeleton (the endosternite), partly chitinoid, but also 

 resembling fibro-cartilage, which lies in the cephalothorax 

 above the nerve-cord, and serves for the insertion of 

 muscles. 



The appendages are — 



i. Small, three-jointed, chelate chelicerce or falces just above the 

 mouth, used in holding prey. 



2. Large, six-jointed, chelate pedipalps. These seize the prey ; 

 their basal joints help in mastication, and in some cases they produce 

 rasping sounds. 



3-6. Four pairs of seven-jointed, non-chelate walking legs. The 

 basal joints of the first two pairs help in connection with the mouth. 



Apparently equivalent to a first pair of abdominal appendages is a 

 small notched plate or operculum which covers or bears the genital 

 aperture or apertures. 



Apparently of the nature of appendages are the comb-like, probably 

 tactile, pectines on the second abdominal segment. 



Six other pairs of abdominal appendages are present in the embryo, 

 but they abort. 



The nervous system consists of a dorsal brain, a. ring round the 

 gullet, and a ventral nerve-cord. The eyes are innervated from the 

 brain, the first six appendages from the collar and the subcesophageal 

 ganglion. Behind the latter there are seven ventral ganglia in the 

 eleventh to seventeenth segments inclusive. There are in scorpions 

 two to six pairs of eyes placed on the carapace. The lateral eyes are 

 simple ocelli, but the median pair are remarkable among Arachnoid 

 eyes, in that, although there is only a single lens, there are numerous 

 retinulje. 



Scorpions seize small animals with their pedipalps, hold them close 

 to the small mouth by their chelicerse, sting them if need be, and suck 

 their blood and juices. The pharynx serves as a suction pump ; a 

 narrow gullet leads to a slight enlargement, into which a pair of 

 salivary glands open ; from the narrow mid-gut several large digestive 

 outgrowths arise, and also one or two pairs of Malpighian tubes ; the 

 hind-gut ends in a ventral anus beneath the base of the sting. The 

 narrowness of the gut may be associated with the fluid nature of the 

 food. The position of the Malpighian tubes shows that here, as in 

 certain Crustacea, they are endoA&cma\ structures as contrasted with the 

 ectodermal tubules of Insects. 



The body cavity is for the most part filled up with organs, muscles, 

 and connective tissue. A pair of coxal glands, perhaps excretory and 

 nephridial, but apparently closed in the adult, lie near the base of the 

 third pair of walking legs. It is stated that in the embryo they open 

 into the body cavity by internal funnels. 



The blood contains amceboid corpuscles and the respiratory pigment 

 hsemocyanin. An eight-chambered heart, within a pericardium, lies 

 along the back of the mesosoma. It gives off lateral arteries from the 

 posterior end of each of its chambers, is continued backwards in a 



