456 INVEETEBBATE MOBPHOLOGY. 



ductive organs show much variety in their arrangeuient, 

 being sometimes paired and sometimes united to a single 

 mass. The single genital orifice is situated far forward, iu 

 some cases even between the basal joints of the second pair 

 of legs. Numerous accessory structures may be associated 

 with the ducts, the receptacula seminis in some forms open- 

 ing to the exterior quite independently of the oviduct, and 

 protrusible organs serving for copulation in the male and for 

 oviposition in the female may occur. The Acarina are as a 

 \ rule oviparous, though a few forms are viviparous. 



Development of the Acarina. — Most of the Acarina whose development 

 has been traced pass through a series of larval stages. While the young 

 embryo is still within the egg and sometimes before the appendages have 

 developed, a cuticular membrane is secreted around it lying between the 

 embryo and the egg-shell. This is the deutovum, and within it further 

 development proceeds. In those forms in which it does not appear until 

 after the appendages are formed a degeneration of these structures takes 

 place, and the egg-shell may also be thrown off leaving the embryo sur- 

 rounded only by the deutovum (Trornbidium). New appendages now 

 appear, and the larva hatches out from the deutovum as a six-legged 

 form, sometimes showing traces of segmentation either iu the thoracic 

 region or in the abdomen. After a certain time a certain amount of de- 

 generation of the tisues occurs (histolysis) and the appendages again dis- 

 appear, a chitinous membrane forming around the now almost spherical 

 body of the larva. A regeneration of the limbs and tissues takes place 

 within this larval membrane, and the nymph is formed, resembling the 

 adult in the number of appendages, but lacking fully-developed repro- 

 ductive organs. A period of rest, and histolysis again occurs, accom- 

 panied by the formation of a third cuticular membrane within which the 

 nymph becomes transformed into the fully-developed and sexually-mature 

 adult or imago, which iinally issues from the membrane. 



This complicated process, it is needless to say, has no phylogenetic sig- 

 nificance, the deutovum indeed being absent in certain forms (Tetrany- 

 ehus), nor does it seem likely that even the six-legged larva is anything 

 but a secondary stage which has been developed within the group of the 

 Acarina. There is no question but that the order represents the culmina- 

 tion of a divergent line of evolution, perhaps from the Pseud oscorpionida 

 and since the separation many of the peculiarities characteristic of the 

 group have been developed. 



Phylogeny of the Arachnida. — There seems little room for doubt but 

 that the Scorpions among living forms represent most closely the ancestral 

 Arachnoids, their segmentation being most perfect and their appendages 

 more numerous than those of other forms. It is through the Scorpions 



