AURELIA. 33 



4. The remaining parts of the digestive (and also in this 

 case circulatory) system include the numerous radial canals 

 and the single circumferential canal. 



(a) Directly beneath each oral arm a per-radial canal is given 

 off, which, at a short distance from the stomach, gives off a 

 branch on either side. The per-radial canal proper usually 

 continues straight to the marginal circumferential canal, without 

 further subdivision, but the two side branches above mentioned 

 in turn subdivide several times. 



(6) From the peripheral wall of each gastric pouch three 

 canals pass toward the margin; the middle one (inter-radial 

 canal) branches somewhat after the manner of the per-radial 

 canals, but the other two (ad-radial canals) continue to the 

 circular canal without further branching. 1 



5. The position of the gastric pouches is made clearly mani- 

 fest by the gonads, which lie on the floor of the pouches, as frill- 

 like structures, horseshoe-shaped, with their open sides toward 

 the mouth. The ova or spermatozoa are shed into the stomach and 

 pass out of the mouth. Embryos in various stages of develop- 

 ment may frequently be found adhering to the oral arms. The 

 sexes are separate. On the sub-umbrellar surface, opposite 

 each gonad, is a little pocket, the sub-genital pit, which opens 

 freely to the outside. Whatever purpose this may serve, it 

 does not function to conduct the genital products to the outside. 



6. Parallel with the inner or concave border of each gonad 

 is a row of delicate gastric filaments. These are supplied with 

 stinging cells, and they may aid in killing live food taken into 

 the stomach. These structures are not present in the Hydro- 

 zoan medusa. 



7. At the marginal extremity of each per-radial and inter- 

 radial canal there is an incision on the edge of the animal, in 

 which there are sensory organs. In each incision find : 



(a) A tentaculocyst in the form of a short, club-like struc- 

 ture containing a prolongation of the circular canal. At its 



1 In most cases the foregoing canals are very evident, but if they are 

 not, they may be injected with water in which carmine is mixed, by insert- 

 ing a large-mouthed pipet into the stomach. 

 3 



