32 



to afford attachment for the muscles of the proximal 

 region of the appendages in this region of the body. It is 

 also useful in supporting and protecting certain portions 

 of the viscera. 



Although at first sight the arthrophragms of the first 

 five post-oral cephalothoracic somites differ in a marked 

 degree from those of the posterior thoracic region, it will 

 be shown that all are built on the same plan. 



Description of a Typical Arthrophragm 

 (Text fig. 4). 



The fourth thoracic arthrophragm (between the 

 fourth and fifth thoracic somites) niay be taken as a type. 

 It is a vertical partition extending inwards at each side 

 from the line of junction of the fourth and fifth thoracic 

 epimera. The portion of the partition in contact with the 

 thoracic sternum arises between the fourth and fifth 

 thoracic sterna. Thus we may distinguish between two 

 kinds of plates, viz., those growing inwards from the 

 epimera — the endopleurites (PL III, fig. 18, ep., also 

 Text figs. 4, 5, 7), and those arising from the inner side 

 of the sternum the endosternites. Each arthrophragm 

 consists, therefore, of an outer endopleurite and an inner 

 endosternite at each side of the middle line. The two 

 endosternites in the arthrophragm under discussion are 

 separated from each other in the middle line by the 

 median plate (fig. 18, med. />.), which is an ingrowth from 

 the median suture present on the last four thoracic sterna. 

 The plates of which the arthrophragm is composed are 

 sometimes known as the " apodemata." 



The Endosternite is irregular in shape and has 

 five principal borders. 



The median border is vertical, and is the part of the 

 endosternite in contact with the median plate. 



