302 Annals Entomological Society of America [Vol. XI, 



apodeme-like projection between the colon and rectum. Like 

 the other circular muscle fibres, they lie in a connective tissue 

 sheath. This sheath forms a continuous tube around the 

 alinientary canal from the anterior end of the pharynx through 

 the posterior end of the colon. 



Longitudinal muscles. New and very strong longitudinal 

 muscle fibres appear with the colon. They lie outside the 

 circular muscles, not inside like the longitudinal muscle's of the 

 ileum. They begin at the point where sections through the 

 ileo-colonic bend first show signs of the approaching con- 

 striction and separation. At their origin, they are inserted 

 on the intima of this region. There are six of these longitudinal 

 muscle fibres, arranged quite regularly in a typical cross section. 

 Three of these muscles appear on the outer (left) side as soon 

 as there is any sign of the constriction referred to above, and 

 three are developed on the inner (right) as soon as the colon and 

 ileum have separated. These muscles grow gradually smaller 

 and are inserted on the intima slightly anterior to the end of 

 the colon. 



Peritoneum and Malpighian vessels. The distal ends of the 

 Malpighian vessels are closely associated with the walls of the 

 colon. The fusion of the Malpighian vessels into two common 

 trunks at the end of their "coelomic" course has been pointed 

 out already (page 298). Each trunk applies itself closely to 

 the wall of the colon, one dorsally and one ventrally, each lying 

 between two of the longitudinal fibres; they join the canal as 

 soon as these fibres have been developed. As soon as the 

 vessels have become appressed to the wall of the colon, their 

 peritoneal sheaths grow out and join, so that a continuous 

 peritoneal coat is formed, which completely surrounds the 

 colon, and encloses the six muscle fibres and the tw^o Malpighian 

 trunks. 



As soon as this tunic is complete, or even before, each trunk 

 (which is not larger than an ordinary Malpighian tube) redivides 

 into three vessels, which almost immediately begin to "migrate" 

 outside the muscle fibres, so as to lie alternately with them. 

 This "slipping" or "migration" extends through about twenty 

 sections of six micra each before it is completed. The condition 

 which results characterizes the greater part of the colon, and in a 

 typical cross-section, one will find lying outside the circular 

 muscles, a layer composed of the six longitudinal niuscle fibres, 



