ANATOMY. 51 



to me that the arrangement of the internal organs is 

 on the whole what might be expected if the hind 

 division of the body be considered to be an abdomen ; 

 the genital organs always lie there, although in some 

 few instances their products are discharged through 

 apertures far forward in the cephalothorax ; this is the 

 case with the male sexual aperture of most Gamasidse ; 

 which is just behind the rostrum, but is not provided 

 with any intromittent organ. A similar position is 

 found in the genital aperture for oviposition of many 

 female Ixodidge ; which is so far forward as to have 

 given rise to the idea that in that family the ova were 

 laid through the mouth-opening. The ventriculus and 

 hind gut are also found in what is usually called the 

 abdomen. The sense-organs are all in the cephalo- 

 thorax. In most soft-bodied Acari the anterior division, 

 or cephalothorax, is capable of considerable movement 

 upon the abdomen as a basis or fulcrum ; special 

 muscles are provided for the purpose. This capability 

 of motion is specially noticeable in Glieyleius and 

 Bdella. In the Oribatidse the two portions of the body 

 are divided by a distinct but small chitinous partition, 

 which no doubt does not extend entirely through the 

 body but leaves a large aperture through which the 

 oesophagus and other organs pass. The cephalothorax 

 and abdomen are often very different in character ; 

 the Phytoptidse are an extreme instance of this. On 

 the whole I am inclined to think that the expression 

 abdomen is fairly used for the portion of the body pos- 

 terior to the constriction between the second and third 

 legs in the Acarina, where that constriction exists ; in 

 accordance with the general practice of acarologists I 

 have used the term in that sense in this book. 



In order to avoid the question and to adopt neutral 

 names Oudemans proposed calling the two divisions 

 of the body in Acarina the " Prosoma " and the " Meta- 

 soma; " one or two acarologists are inclined to follow 

 him. 



Kramer and some other acarologists following him 



