450 THE INVERTEBRATA 



which are masticatory in function (gnathobases). The walking legs 

 are six -jointed and end in double claws. Between the basal joints of 

 the last pair is a plate, the metasternite , which represents the fused 

 sterna corresponding to these limbs; the sterna of the other pro- 

 somatic segments are not represented. At the beginning of the pro- 

 soma there is in the embryo a pregenital segment with two limb 

 rudiments. This disappears without leaving a trace in the adult. The 

 two succeeding segments bear appendages: (i) the genital operculum, 

 a small plate covering the openings of the genital ducts, which is 

 formed by the union of two rudiments of appendages; (2) the pectines, 

 flap-like structures attached by a narrow base with a distal border of 

 chitinous spines like the teeth of a comb. They are tactile in function 

 and derived from embryonic limb rudiments. There are no other ex- 

 clusively sensory organs (except the eyes) on the body of the scorpion, 

 but there are sense hairs scattered over the surface and thicker on the 

 pedipalps than elsewhere. 



The lung books are found on segments 3-6 of the mesosoma. The 

 7th segment is without any external segmental organs. As has been 

 already mentioned, there are, in the embryo, seven pairs of meso- 

 somatic appendages (Fig. 328 D), those on the embryonic pregenital 

 segment and on the six succeeding segments. Of these the 4th-7th 

 never develop to more than papillae, but folds develop on their 

 posterior surface and the skin behind is tucked in to form the lung 

 sacs. When the sacs are complete, the folds become the leaves of the 

 lung book. In the internal space of these folds, the blood circulates 

 and is presumably aerated; it contains the respiratory pigment, 

 haemocyanin. The circulatory system of the scorpion is remarkably 

 complete (Fig. 333). The heart consists of seven chambers (in the 7th- 

 13th segments), into each of which a pair of ostia opens and from each 

 there leave a pair of lateral arteries. In addition, there is an anterior 

 and a posterior aorta, the former dividing into many branches in the 

 cephalothorax, and one of these passes backwards as a supraneural 

 artery. The arteries end in tiny vessels and many of these communicate 

 with the special ventral sinus, which supplies blood to the lung books. 

 Muscles run from the roof of this to the floor of the pericardium, and 

 when they contract the ventral sinus enlarges and draws venous 

 blood into it. When they relax, blood is forced into the lung books, 

 whence it is returned to the pericardium by segmental vessels. 



A minute mouth opens into the pharynx which is suctorial, with 

 elastic walls which can be drawn apart by muscles. A short oeso- 

 phagus succeeds, and into this open the salivary glands. The endo- 

 dermal mid gut is long and narrow and receives throughout its course 

 several pairs of ducts which lead from the digestive glands. These 

 together form a bulky mass, filling up the dorsal part of the meso- 



