414 PHYLUM ARTHROPODA 



The appendages are — 



1. 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 sub -oesophageal 

 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 

 simpler than the median pair, and the type is in some ways inter- 

 mediate between the simple eye and the compound eye. There is, as 

 in ocelli, a single crystalline -lens -like portion, below which there are 

 a few groups of retinal cells. Each group has five cells, and the outer 

 part of each cell is pigmented. There is no crystalline cone. 



Scorpions seize small animals with their pedipalps, hold them close 

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

 their blood and juices. The pharj^nx 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 out- 

 growths arise ; 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 so-called Malpighian tubes of Buthus 

 europcBus are really the ducts of the liver. 



The cavity of the body 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 amoeboid corpuscles and the respiratory pigment 

 haemocyanin. 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 

 posterior aorta, and forwards in an anterior aorta. The latter supplies 

 the head and divides into two branches, encircling the gullet and 

 reuniting in a ventral artery above the nerve-cord. From capillaries 

 the blood is gathered into a ventral venous sinus, is purified in the 

 lung-books, and thence returns by veins to the pericardium, finding 

 its way by valved lateral openings (ostia) into the anterior end of each 

 heart -chamber. 



