OF TREPANGS. 191 



several intervals which are embraced by the pairs of muscles. It 

 will be seen presently that each of these intervals is occupied by 

 one of the longitudinal canals (c) of the water circulatory sys- 

 tem, to which we will now return. The canal which corresponds 

 with the one which encircles (fig. Ill, aq) the throat of the star- 

 fish, is suspended from the points of the forks (fig. 114, /) of the 

 buccal plates, and pursues a rather irregular course (r) in its 

 circuit. At each fork a canal passes from it directly forward to 

 and into the five tentacles which lie in the same trend, and at 

 the same time pours its contents into a circular passage (aq B ) 

 which runs along the base of these tentacles, and thus indirectly 

 supplies the ten alternating ones. From each of the five points 

 where the canals from the forks enter the tentacles, a simple 

 canal (figs. 114, 115, aq 1 , aq*, aq 4 to a# 9 ) arises which extends 

 backwards along the side of the body, in the furrow that sepa- 

 arates each pair of muscles (fig. 116, m, in 1 ) from one another. 

 There are no external tubular feet like those of the starfish, and 

 consequently no lateral branches from the longitudinal canals, 

 except at their anterior points of origin, where they connect more 

 or less directly with the cavities of the tentacles. The latter are 

 all that remain, in this reducing process, to represent the numer- 

 ous, tubular, external appendages of the Starfishes, Sea-urchins, 

 and the lower ranks of Trepangs. The madreporic canal (figs. 

 114, 115, m to me) arises from the circular canal (r) in the supe- 

 rior middle line of the body, between the two upper forks of the 

 buccal plates. It is bent upon itself several times so as to form 

 a thick, convoluted, elongate mass, and is terminated (m) by the 

 homologue of the madreporiform plate of the starfish, but, unlike 

 that of the latter, is wholly included within the body cavity. 



The blood circulation is carried on in a system of vessels 

 which run along the upper and lower edges of the intestine, and 

 connect with each other by multitudes of little cross branches. 

 The latter, as they traverse the intervening space, are inter- 

 locked with the twigs of the respiratory branches, and in this 

 way the blood is aerated. The principal trunk of this system is 

 a thick tube (/*, A 1 ) which courses along the back of the stom- 



