224 ACTINOZOA. 



the other pair, parallel to these, being situate 

 midway between them and the opposite^ or oral, 

 margin. For, in this strangely aberrant form, the 

 course of the axial system corresponds with the 

 short mid-axis of the ribbon, the apparent width 

 of which represents, therefore, the true height of 

 the animal, whose breadth answers to the length 

 of the ribbon, and antero-posterior diameter to its 

 thickness. On this account, it will be convenient 

 to speak of two of the ctenophoral canals as mar- 

 ginal, the two others being medial. There are 

 four radial vessels, two on either side, each of 

 which divides into a single pair of branches, com- 

 municating with the ctenophoral canals. The two 

 branches of each radial canal are very unequal in 

 length, and run in opposite directions, the shorter 

 branch soon becoming continuous with a marginal 

 canal, while the longer branch trends parallel to 

 the sides of the digestive cavity, turning round 

 abruptly to open at right angles into one of -the 

 medial canals, as soon as it has reached its level. 

 At each extremity of the ribbon the marginal and 

 medial canals anastomose with one another and a 

 long vessel running parallel to their course along 

 the oral margin of the body. At its opposite 

 extremity, near the mouth, this canal unites with 

 its fellow of the other side, w^here both are joined 

 by a pair of paragastric canals, which Milne 

 Edwards has figured as running in an antero- 

 posterior direction, parallel to the digestive sac. 

 A second, or inner, pair of these canals has not 

 yet been observed. 



There can be little doubt that the paired medial 

 and marginal canals of Cestum represent the eight 

 ctenophoral vessels of other genera of the order. 



