XI 



PHYLUM ARTHROPODA 



643 



operculum, the pectines, and the two following segments. The 

 first ganglion behind the suboesophageal ganglion appears in the 

 eleventh segment (reckoning the cephalothorax as made up of six) ; 

 behind which a ganglion occurs regularly in each segment as far back 

 as the fourth of the post-abdomen. 



The organs of special sense are the eyes and pectines. The 

 lateral eyes (Fig. 566) are similar in character to the simple eyes or 

 ocelli of Insects. The two larger central eyes (Fig. 567) differ from 

 them in having the retinal cells arranged in groups as in the com- 

 pound eye, but resemble them in the presence of a single cuticular 

 lens. 



Reproductive organs. In the male the testes consist of two 

 pairs of longitudinal tubules united by cross branches. These 

 are connected with a 

 median vas deferens, the 

 terminal portion o f 

 which, provided with 

 accessory glands, is modi- 

 fied to form a double 

 penis ; its external open- 

 ing is just behind the 

 operculum, as already 

 noticed. There is an un- 

 paired ovary, which is 

 made up of three longi- 

 tudinal tubules with 

 transverse connecting 

 branches ; the oviducts 

 open on the operculum. 



Scorpions are v i v i - 

 parous. 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 

 occurs in the follicle 



FIG. 548. 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.) 



or 



of the oviduct. Fertilisation either 

 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 pole 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. 548) corresponding to that of the Insect. A longitudinal 

 groove which appears on the surface of this may be regarded as 

 representing an elongated blastopore (Fig. 548, A). The cells of the 



