VI 



ARACHNOIDEA ONTOGENY 



537 



certain extent within the mother body, so that the young is hatched soon after the 

 laying of the egg. Most Acarina, however, are oviparous. The eggs or embryos 

 collect often in great numbers in the expanded oviducts, which then function as 

 uteri. 



Linguatulidse. Female Apparatus (Figs. 375 E, 378). The ovary is a long un- 

 paired tube, beset with ovarian follicles, which runs through the body over the 

 intestine in a longitudinal direction. It is continued 

 anteriorly into 2 oviducts, which surround the oeso- 

 phagus, and under it enter the anterior end of the 

 unpaired vagina. This serves at the same time as 

 uterus, the first embryonic development of the eggs 

 taking place in it. The vagina is an unusually long 

 tube which runs backward with many windings, 

 accompanying the intestine, and is often filled with 

 several hundreds of thousands of eggs and embryos ; 

 it opens outward through a female genital aperture 

 close to the anus. The ducts of 2 long receptacula 

 seminis, which lie at the 2 sides of the mid -gut, enter 

 the most anterior end of the vagina at the point where 

 the oviducts join it. 



Male Apparatus (Fig. 376, E). The testes are 

 paired or unpaired tubes placed like the ovary. 

 The testis or testes are continued anteriorly into 

 an unpaired efferent division, which has been 

 regarded as a seminal vesicle. This vesicula semi- 

 nalis divides anteriorly into 2 canals, the vasa 

 deferentia, which encircle the oesophagus. Each vas 

 deferens ends in a male copulatory apparatus. The 

 male genital aperture common to the 2 copulatory 

 apparati lies, in contradistinction to that of the 

 female, in the anterior portion of the body, between 

 the 2d pair of hooks. Corresponding with the recep- 

 tacula seminis of the female there are in the male 2 

 blind tubes running backward, these are apparently 

 organs for propelling the semen, and enter the 2 sperm 

 ducts. The end of each sperm duct enters a very long 

 chitinous cirrus, which at a time of rest is rolled up 

 in a special sac. 



IX. Ontogeny. 



FIG. 378. Female of Pentas- 

 tomum taenioides at the time of 

 copulation, with the viscera (after 

 Leuckart). h, Hooks ; oe, oeso- 

 phagus ; rs, receptacula seminis, 

 one of which is still empty ; rf, 

 gut ; or, ovary ; va, vagina. 



We can only bring forward a few facts concerning 

 the ontogeny of the Arachnoidea, chiefly such as are 

 of most importance from the point of view of compara- 

 tive anatomy. 



1. The segmentation is in the main that of the centre- or mesolecithal eggs. A 

 blastoderm is formed covering the yolk, in which, however, merocytes remain. The 

 formation of the germ layers and of the rudiments of the most important organs 

 proceeds, as in other Arthro2)oda, from a blastoderm plate, which may be called the 

 embryonic rudiment. In the Scorpionidce, however, the egg seems to be meroblastically 

 telolecithal, and the furrowing takes a corresponding course, so that no blastoderm 

 enveloping the yolk on all sides is formed, but a germ disc is developed at one pole 

 of the egg. 



