No. 2.] AA'ATOJlfV OF BDELLODRILUS ILLUMINATUS. 515 



intestinal enlargement remains throughout its entire length 

 strictly a part of the sinus, with which it is in continuous com- 

 munication ; but the dorsal one, which has a similar relation 

 posteriorly, begins in the fifth somite to have the character of 

 a distinct vessel, which has more or less complete walls and 

 rises above the level of the sinus, from which it may be en- 

 tirely free for a short distance, but again open into it through 

 a cleft (Fig. 12). Perforating the septum 4-5, from which its 

 muscular walls receive accessions, it becomes a distinct muscu- 

 lar tube, still imbedded amongst the chloragogue cells, and, 

 increasing in diameter, passes obliquely down the right side of 

 the intestine, with which it loses all connection in the third 

 somite, and becomes a regularly pulsatile tube — the so-called 

 heart. The heart thus formed is thrown into a conspicuous 

 loop, which accommodates itself to changes in the animal's 

 length. It lies to the right of the oesophagus, and in the first 

 somite rises once more to a dorsal position and gives off the 

 fifth pair of vascular arches ; anterior to which it continues 

 into the head as the dorsal trunk. 



At the place of origin of the heart from the blood sinus cer- 

 tain chloragogue cells extend into its lumen, and give rise to 

 a remarkable chain of valve cells, which retain all of the visible 

 characters of the chloragogue cells. This rod is chiefly con- 

 fined to the ventral wall of the heart, to which it is more or 

 less closely bound. The inter-cellular space described by 

 Voigt is well marked in this species, and may be seen to in- 

 crease and decrease in size with the expansion and contraction 

 of the heart. Occasionally in sections (Fig. 14, b) the cells 

 form only a thin wall around a conspicuous lumen. In some 

 cases also there are indications of a special muscular layer sur- 

 rounding the cells (Fig. 14). They are also attached here and 

 there to the walls of the heart by delicate threads (Fig. 7, //). 

 The rod is continuous throughout the greater part of the heart 

 and may even extend into the dorsal vessel of the head (Fig. 9). 

 Well fitted to serve as valves, the peculiar arrangement of the 

 cells permits them to act also in the direct propulsion of the 

 blood. The cells present more flattened surfaces anteriorly, 

 and more sloping ones behind, and swinging forward as the 



