SEA-FISIlERlES LABORATORY. 329 



pleurite. Also at the edge of the foramen bounding the 

 pleuro-sternal muscle chamber this endosternite is 

 fused in front to the fifth and behind to the seventh 

 endosternite (7.). 



The seventh thoracic arthrophragm (Text fig. 5, D.) 

 lies between the seventh and eighth thoracic somites. 



The endosternite is inclined at an angle of 50° to the 

 vertical, the upper border being anterior. It is almost 

 completely covered by the overhanging sixth endosternite. 

 The median border is very deep and the sternal canal is 

 very small. The dorsal border is almost level, and partly 

 bounds the ventral side of the pleuro-sternal muscle 

 chamber. The endosternite does not completely surround 

 this chamber as in the two previous arthrophragms. The 

 sternal border is inclined at a considerable angle to the 

 horizontal. The apodemal foramen is small. 



The plane of the endopleurite is almost at right 

 angles to that of the endosternite. At the junction of the 

 endosternite and 'endopleurite this arthrophragm fuses in 

 front with the sixth endopleurite (o.), and where the 

 endosternite borders the pleuro-sternal muscle chamber 

 there is a fusion with the sixth endosternite (p.). 



The eighth thoracic arthrophragm (PI. Ill, fig. 18, 

 e. st. 13) lies at the posterior end of the last thoracic 

 somite. In this somite there is no separate epimeron. 

 It is probably fused with the sternum. The arthro- 

 phragm, therefore, shows no division into endosternite 

 and endopleurite. It may be accepted, however, that this 

 arthrophragm represents the fused endosternite and endo- 

 pleurite. It consists of two halves, which are separated 

 in the median line by the posterior end of the median 

 plate. This arthrophragm is practically horizontal, and 

 was designated the " sella turcica " by Milne-Edwards. 

 As already stated, the last arthrophragm fuses in front 



