2 22 ALUS. [Vol. XVIII. 



occupies a position ventral to the horizontal muscle septum cor- 

 responding exactly to that of the third pocket dorsal to that sep- 

 tum. It can accordingly be called the ventro-median pocket. In 

 the anterior segments it gradually diminishes in size, and even in 

 the tenth muscle segment has largely disappeared. The fifth 

 pocket, when found, is only slightly developed, is directed back- 

 ward, lies internal to the ventro-posterior angle of the surface 

 septal line, and can be called the ventro-posterior pocket. These 

 several pockets all lie free in the muscle fibers of the adjoining 

 segments, no ligamentous attachments of any kind being found in 

 connection with them. The muscle fibers that have their attach- 

 ments on the outer surface of the point of each pocket, are, how- 

 ever, more tendinous at their insertion than the other fibers of the 

 segment, a tuft of fibers usually being found associated with the 

 point of each pocket. 



The mesial edges of the septa have a course approximately sim- 

 ilar to that of their superficial edges, but they lie in front of the 

 latter edges ; in certain parts of their course considerably in front 

 of them. In its mid-lateral part, each septum follows and is at- 

 tached to the corresponding horizontal rib, that rib giving attach- 

 ment also to the horizontal muscle septum. Ventral to this latter" 

 septum the mesial edges of the intermuscular septa follow, and are 

 attached to, the ventral ribs of the vertebrae to which they belong. 

 They do not, however, ' in those anterior trunk segments that 

 alone were examined, follow those ribs to their distal ends. Short- 

 ly before reaching the distal end of the rib to which it belongs, 

 each septum turns sharply forward, and, running downward and 

 forward, crosses the next anterior rib close to its distal end. 



Dorsal to the horizontal muscle septum, the mesial edge of each 

 intermuscular septum runs upward and backward along the lateral 

 surface of the vertebra to which it belongs, its line of attachment 

 being along the ridge that represents the base of the spine-like 

 part of the dorsal arch of the vertebra. It then passes onto the 

 spine of the arch, and continues upward and backward nearly to 

 its dorsal end. There it turns backward and slightly upward and 

 crosses the distal end of the next posterior dorsal arch. At this 

 point it lies close to the next posterior septum of the trunk, and 

 is connected with that septum by fibrous tissue, the intervening 



