24 ARACHNIDA. 



cells yield the blood-corpuscles. A similar process has been observed in the 

 formation of the heart of the Araneae (see Figs. 45-47, pp. 86-89). 



The description of the origin of the heart given by Kowalevsky and 

 Schulgin is not altogether easy to reconcile with that given by Laurie. The 

 statements of the former seemed so definite that we felt obliged to follow them ; 

 but, on the other hand, the observations of Laurie agree better with the 

 processes which take place in the Araneae. It is, in fact, impossible to obtain 

 a clear idea of the whole process from the works under consideration. According 

 to Laurie, it appears as if the dorsal part of the mesoderm had already split 

 when the formation of the heart begins, in which case this organ would 

 develop as in the Araneae. We are, moreover, disposed to regard the process 

 as resembling that in the Annelida, and to imagine a delamination of the 

 mesoderm to form the heart ; this organ, however, is thought to arise somewhat 

 differently in the Araneae (p. 88). 



Kowalevsky and Schulgin distinguish an endothelium and a muscle-layer 

 in the heart, both arising from the mesoderm. During the differentiation of 

 these layers, the ostia appear in the wall of the heart. The alary muscles 

 form from the mesoderm, and a layer of mesoderm-cells appears around but 

 at a little distance from the heart, forming a continuous envelope to it ; this 

 is the pericardium. 



The coelom of the Scorpiones, up to the time when the heart 

 forms, closely resembles that of the Annelida. It consists at first 

 of separate divisions formed in the primitive segments. The anterior 

 and posterior walls (dissepiments) of the latter are broken through, 

 but the cavities themselves are retained for a time, and are lined 

 by the coelomic epithelium ; they thus represent a true coelom. 

 At the time when outgrowths of the splanchnic layer extend in 

 between the lobes of the liver, this is, according to Laurie, still 

 the case. These cavities then become filled with cells, which 

 doubtless arise during the disintegration of the wall of the primitive 

 segments. The somatic layer undergoes further differentiation, the 

 body-musculature forming out of it. The coelom will be further 

 described in connection with the development of the Araneae, in 

 which it is better known than in other Arachnids. 



F. The Coxal Glands. 



A complicated coiled gland is found on each side of the cephalo- 

 thorax in the Scorpiones, opening, in the young, on the base of the 

 third ambulatory limb (Fig. 14, A). At its earliest stage, this gland 

 is described by Laurie as a simple, straight tube, which runs forward 

 from its aperture at the base of the third ambulatory limb in the 

 somatic layer of the mesoderm, and communicates with the coelom ' 

 through a funnel-shaped aperture. The tube becomes closely coiled 

 later, finally forming the glandular mass which is found in the adult. 

 The external aperture was still evident in the young when ready for 



