DEVELOPMENT OF THE EXTERNAL FORM. 



11 



invaginations form, the abdominal limbs themselves gradually de- 

 generate. In the embryo depicted in Fig. 8, the abdominal limbs 

 can still be recognised, while, in Fig. 7 C, the four pairs of stigmata 

 are already visible. 



The origin of the lungs as invagina- 

 tions on the dorsal side of the abdominal 

 limbs is a point of importance in com- 

 paring the Scorpion es with Limulus, 

 since this ontogenetic feature is refer- 

 able to the drawing of the branchial 

 lamellae into the body. Although 

 we feel inclined to adopt this point 

 of view, it appears to us that the 

 proofs brought forward by Laurie in 

 support of this important point are 

 not sufficient, and that the whole 

 subject requires more careful and 

 thorough investigation than it has 

 received in his treatise. It cannot 

 be denied that the relation of the 

 lung- invaginations to the abdominal 

 limbs is very striking. This is evident 

 from Fig. 8, after Metschnikoff, al- 

 though this author remarks that the 

 lung - invaginations are not derived 

 from the limbs, but arise at tbe points 

 where the abdominal limbs have dis- 

 appeared. 



Fig. S.— Embryo of Euscorpius italicus (after 

 Metschnikoff). 1-U (p), the four pairs of 

 limbs; op/j-apyj, abdominal limbs [III- 

 VII, Braver] ; bg, ventral chain of ganglia : 

 ch, ehelicerae ; m, mouth ; pab, post-abdo- 

 men ; pcd, pedipalps. 



Behind the last stigmata-bearing segment another pre-abdominal 

 segment develops, this being followed by the post-abdomen of five 

 true segments and the telson. The post-abdomen is still bent round 

 ventrally (Fig. 7 G). The telson has developed at its end, and two 

 ectodermal invaginations give rise at its point to the paired poison 

 glands, -which in the adult still open through two apertures at the 

 tip of the telson. The anus appears as an ectodermal invagination 

 at the end of the last true segment of the post-abdomen. 



By means of the modifications just described, the embryo attains 

 the general form of the adult, these external changes having been 

 accompanied by development in the internal organs, which will be 

 described later, and in the covering of the body, chief among these 

 being the secretion of the chitinous cuticle. At the time of hatching, 

 the young Scorpion raises its post-abdomen (tail) over its back, thus 

 completing its resemblance to the adult. 



