XI 



PHYLUM ARTHROPODA 



Scorpions are viviparous. The eggs, which are spherical or oval, 

 and in most species contain a large amount of food-yolk, lie in a 

 follicle formed of a diverticulum of the oviduct. Fertilisation 

 either occurs in the follicle or after the egg has escaped into 

 the oviduct. The further development takes place in the oviducts : 

 and, when born, the young Scorpion differs from the parent very 

 little save in size. 



Development. The segmentation is of the type to which the 

 term discoidal is applied. On one side are formed a number of cells 

 in the form of a one-layered disc or cap, which gradually spreads over 

 the yolk. On this appears a thickening the ventral plate (Fig. 538) 

 corresponding to that of the Insect. A longitudinal groove which 



FIG. 538. Three surface views of the ventral 

 plate of a developing Scorpion. A, 

 before the appearance of segments ; B, after 

 five segments have become formed ; C, after 

 the appendages have begun to be formed. 

 (From Balfour, after Metschnikoff.) 



FIG. 539. Embryo of Scorpion (Euscor- 

 pius italicus), later stage. ap. 

 II. VI., abdominal appendages; ck. 

 chelicerse ; p. 1 4, legs ; ?. mouth ; 

 ped. pedipalpi ; pab. post-abdomen, 

 (From Korschelt and Heider, after 

 Metschnikoff.) 



appears on the surface of this may be regarded as representing 

 an elongated blastopore (Fig. 538, A). The cells of the blastoderm 

 of the ventral plate become divisible into three layers ectoderm, 

 endoderm, and mcsodcrm. The mesoderm undergoes division into 

 a series of masses which are hollowed out to form the primitive 

 segments (B) and their cavities. Embryonic membranes -serosa 

 and amnion are formed as in the Insects. When about ten seg- 

 ments- have become distinguishable, the rudiments of appendages 

 (Fig. 538 C, and Fig. 539) appear in the form of hollow processes 

 of the segments on either side of the middle line. Behind the 

 rudiments of the thoracic limbs appear a series of six pairs of 

 abdominal appendages (ap* I. VI.); the place of the first of these 



