44 Invertebrate Zoology. 



Draw. 



J 



Open the animal along the mid-dorsal line, and, after 

 gently pinning the cut edges apart, note the regular seg- 

 mentation of the body-cavity. The transverse partitions or 

 dissepiments correspond in position to the constrictions 

 between the successive rings or metameres. 



The Digestive System. Cutting through the dissepi- 

 ments, partially reflex the body-walls from the alimentary 

 canal. The mouth will be found to lead into a protrusi- 

 \&& pharynx which, in turn, leads into a somewhat smaller 

 and less muscular crop. The crop, separated from the 

 pharynx by a marked constriction, is in turn separated in 

 the same way from the elongated and thin-walled stomach- 

 intestine. The latter extends the remaining length of the 

 animal. Do you find muscles connecting the pharynx to the 

 inner walls of the peristomium ? 



These muscles are the "protractors" Their points of 

 origin and insertion should be clearly established. The 

 lesser protractors are also longitudinally arranged, but are 

 not attached otherwise than to the walls of the pharynx. 

 The retractors, unless the pharynx is extended, will be ob- 

 served as fine fibres in the region of the first and second 

 metameres. Determine their distribution. 



The Crop. Are there muscle fibres extending from the 

 crop to the body- walls? Do you find a gland on each side 

 of the crop? Trace its duct. How do the walls of the 

 crop compare with those of the pharynx ? 



The Stomach-Intestine. Is this portion of the ali- 

 mentary tract regularly constricted throughout its entire 



