28 LABORATORY DIRECTIONS IN 



6d. Blood System. The dorsal blood vessel may be seen in the 

 living worm. In favorable specimens it may be seen to pulsate. In the 

 dissected worm it is found imbedded on the dorsal side of the digestive 

 tract. Follow the dorsal vessel forward. In somites 7 to 11 inclusive, 

 will be found certain paired, tube-like red bodies (variously colored in 

 preserved worms), the hearts, which are connected with the dorsal blood 

 vessel. The hearts extend ventrad, forming semicircular loops on each 

 side of the digestive tract. They unite below with a ventral blood vessel, 

 which extends backward along the ventral side of the digestive tract. 

 If the hearts cannot be seen carefully dissect away the remaining portion 

 of the very prominent septa which obscure the hearts and other organs 

 of somites 7 to 12. The ventral vessel will be seen later in cross-sections. 

 In life the hearts propel the blood from the dorsal to the ventral vessel. 

 Smaller vessels are found throughout the body. Some of the more promi- 

 nent of these may be found in each segment back of the hearts connecting 

 the dorsal blood vessel with the body wall and the intestine. What is 

 the function of the blood system? How is this function served in Hydra? 



6e. Digestive System. This consists of a tube extending through the 

 whole length of the body. It is modified into various parts which may 

 be readily found. Beginning at the anterior end these are taken up in 

 order. 



The mouth has already been found. It leads into the mouth cavity 

 or buccal pouch in the first three somites. Be careful not to injure the 

 brain, a whitish bi-lobed structure situated on the dorsal side of the mouth 

 cavity in somite 3. 



The pharynx is the thick-walled portion following the buccal pouch. 

 It extends to about the 7th somite. The walls are firm and muscular. 

 Test the consistency of this structure with your dissecting needle. 



The esophagus is a long slender portion behind the pharynx. It is 

 partly covered by the hearts and reproductive organs, and in the 

 anterior part by heavy septa. The hearts and reproductive organs must 

 not be removed or injured but the reproductive organs may be carefully 

 turned aside in order to reveal the esophagus. 



The crop is an enlargement following the esophagus. It is situated 

 directly behind the last pair of seminal vesicles in somites 15 and 16 

 (usually). Feel of it to determine whether it is thick or thin walled. 



The crop is followed by the whitish gizzard. Feel of this organ to 

 determine whether it is thick or thin walled. Behind the gizzard, the 

 intestine extends to the posterior end of the worm where it opens to the 

 exterior by means of the anus. 



Compare the digestive system of the earthworm with that of Hydra. 



6/. Excretory System. Find, with the dissecting microscope if neces- 

 sary, a pair of coiled tubes in each somite except a few at the anterior and 

 posterior ends. They are located between the septa and partly beneath 



