52 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



The enclosternites and endopleurites formed in the manner just 

 described are entirely intersegmental. A secondary infolding of the 

 pleural wall occurs, however, in the fourth, fifth, sixth, and seventh 

 thoracic segments. To this infolded structure, which is strictly intra- 

 segmental, I have given the name of secondary endopleurite. No cor- 

 responding infolding occurs in the sternal parts of these segments. 

 The secondary endopleurite is firmly attached at its inner margin to 

 the anterior surface of the membrane encasing the promotor of the 

 fifth pereiopod. The remotor muscle always finds its origin behind 

 the secondary endopleurite, while some of the branches of the depres- 

 sor and levator do so likewise in certain segments. This indicates 

 that these partitions are in truth only secondary, since the remotor of 

 a particular segment would not arise outside its own segment. 



The endoskeletal partitions of the last five segments of the thorax 

 present an interesting complexity due to the overdevelopment of the 

 fifth pereiopod, as I have already noted. The muscle attachments of 

 this pereiopod have been increased by the forward prolongation of a 

 branch of the promotor muscle through the three preceding segments. 

 The pocketlike membrane that encases this part of the muscle serves 

 as a place of attachment for the several endopleurites where they 

 meet the endosternites, as well as for the secondary endopleurites, and 

 these attachments hold it firmly in place to resist the heavy pull which 

 the muscle exerts upon it. The anterior termination of this pro- 

 longation may be seen upon the posterior wall of the fourth thoracic 

 segment, where it appears as an oval, semi-transparent window partly 

 separating the endopleurite and endosternite lying between the fourth 

 and fifth thoracic segments. 



Although the median plate extends forward as far as the endoster- 

 nite separating the first and second pereiopods, it serves exclusively 

 as a place of origin for branches of the four basal muscles of the 

 telopodite of the fifth pereiopod. Some part of each of these muscles 

 originates upon the median plate, although none of the muscles origi- 

 nates entirely upon it. 



The third maxilliped and the first pereiopod bear a pair of gills, 

 which lie side by side in the gill chamber. The second maxilliped like- 

 wise possesses two gills, one of which lies in the extreme anterior part 

 of the gill chamber in front of the gills belonging to the pereiopods, 

 and which can be distinguished from them only by its smaller size and 

 its anterior position. The other gill of the second maxilliped lies at 

 right angles to the first, extending outward and backward from the 

 anterior corner of the gill chamber. The second and third pereiopods 

 each possess a single gill. The first maxilliped and the fourth and 

 fifth pereiopods lack gills. 



J 



