viii AETHEOPODA 241 



All the nerve ganglia of the prosoma and the first pair of the 

 abdominal ganglion fuse to form the suboesophageal ganglion. 



The heart is formed, just as in the spider, l>y the meeting of the 

 dorsal ends of the coelomic sacs of opposite sides. A semicircular 

 plate of cells is detached from the apex of each of the two coelomic 

 sacs, and the two grow together to enclose a blood space which is 

 the heart. Brauer has described in detail how the pericardium is 

 formed. The two semicircular plates become completely detached 

 from the coelomic sacs of which they once formed part, these latter 

 shrink away from the heart and meet beneath it, and the space 

 which is left between heart and conjoined coelomic sacs is the 

 pericardium. The coelomic sacs then lose their cavities by a process 

 which consists in the development of fibrils crossing their lumen, 

 and the swelling up and rouudiug of the cells forming their walls, 

 and the comparatively solid plate of connective tissue which results 

 forms the pericardial septum or floor of the pericardium. A similar 

 process then occurs in the remaining parts of the coelomic sacs, and 

 the only portions of their cavities which persist are found in the 

 coxal glands, and presumably in the genital organs, though the 

 origin of these latter was not traced. 



What little is known of the course of development in other 

 orders of Arachnida fits in well with what is known of the scorpion, 

 spider, and Limulus. The formation of sheets of cells acting as 

 protective coverings to the developing embryo is not known to 

 occur except in the scorpion. In all cases, however, an indication 

 of the primitive cumulus at the hinder end of the primitive streak 

 can be made out, and in the Pedipalpi, according to Schimkewitsch, 

 the formation of mesoderm takes place from the cumulus alone. 

 Thus the cumulus cannot merely represent the genital rudiment 

 but must represent the primitive streak of Peripatus, i.e. the 

 obliterated section of the blastopore which occurs behind the anus 

 in that animal. For a similar reason the primitive streak of 

 Arachnida must represent the open portion of the blastopore of 

 Peripatus. 



In the Acarina or Mites the young are hatched in an imperfect 

 form in which only three pairs of walking legs are developed. After 

 living in this state for some time they moult a thick cuticle, which, 

 however, remains surrounding them like a second egg-shell, called the 

 deutovum, inside which further development takes place and the 

 missing fourth pair of legs make their appearance. This reduction 

 of a series of homologous organs to a smaller number, in accordance 

 with the minute size of the embryo when hatched, is entirely in line 

 with what we have learnt of larval modification amongst Crustacea. 



The Pentastomida, which, even when adult, are parasitic in the 

 nasal cavities of the dog, are supposed to be the extreme limit of 

 degeneration in Acarina. The larva, which encysts itself in the 

 connective tissue of rabbits, sheep, etc., exhibits the rudiments of 

 two pairs of appendages. 



VOL. i it 



