218 



The gastro-intestinal typhlosole enlarges in the intestine a short distance from 

 the stomach, and coils to the right, forming the large coiled typhlosole of the intesline 

 {c.t.i., figs. 77, etc.). It then again resumes its former size and continues to the sharp 

 bend in the intestine. From here to the anus the typhlosole is entirely absent. 



Figure 78 shows the dilation of the intestine to two or three times its diameter 

 elsewhere for the accommodation of this unique structure. In this same figure can 

 be seen the line where the intestinal wall folds in to form the typhlosole. At the distal 

 end of the rectum are a few strands of sphincter muscle. The intestine is ciliated 

 throughout. 



Sigerfoos gives a similar description, but in his species a small typhlosole extends 

 throughout the whole intestine. Deshayes and Quatrefages describe the intestine 

 as being of uniform diameter throughout, indicating the absence of a coiled typhlosole. 

 At the same time the intestine is longer and much convoluted, though its general 

 course through the body is similar. These two distinct types of intestine are easily- 

 observed, and should prove of systematic value. The consideration of such internal 

 characters may shed some light on the badly tangled taxonomy of the group. 



The intestine is always found to be full of wood chips and the remains of diatoms 

 and other plankton organisms. In a piece of intestine removed from a small living 

 Teredo the contents were observed to move posteriorly about one millimeter in two 

 minutes as the result of ciliary action. 



THE CIRCULATORY SYSTEM 



The pericardium of Teredo is much elongated, extending from the posterior 

 adductor muscle to a point some distance back of the posterior extremity of the 

 caecum. The heart, which occupies a median position in this cavity, is composed of 

 two elongated auricles and an elongated nuiscular ventricle. In a living specimen 

 the pulsations of the heart can be plainly seen through the transparent tissues of the 

 body wall. 



Each of the auricles receives from behind one of the paired efferent branchial veins. 

 From these aerated blood from the branchial region enters the auricles, whence it 

 passes to the ventricle and is pumped into a large aorta. This vessel passes ventrally 

 through the floor of the pericardium, and immediately divides into three branches. 

 As has been previously explained, the morphological relations of the circulatory 

 system have been reversed by the partial rotation of the heart and pericardium around 

 the posterior adductor muscle. Hence, what is morphologically the anterior aorta 

 passes backward to carry a blood supply to the visceral mass. A second vessel, morpho- 

 logically the posterior aorta, passes forw^ard, under and around the posterior adductor 

 muscle, and enters the mantle. It then continues backward as the pallial artery to the 

 base of the siphon, where it divides, furnishing a blood supply to each. The third, which 

 may be termed the caecal artery, drops immediately and enters the anterior end of 

 the caecal typhlosole. After passing through the typhlosole, the blood flows into the 

 sinuses in that part of the body. 



The circulation of blood in the various parts of the digestive tract is still to be 

 worked out in detail, but one or two major points may be mentioned. The whole 

 tract, including the livers, is evidently bathed in blood carried in sinuses with connec- 

 tive tissue walls (Pelseneer, 1906). The typhlosoles, however, have a definite arterial 

 supply. That of the caecal typhlosole has been described. A small branch from the 

 posterior aorta (fig. 77) supplies the gastro-intestinal typhlosole at the bend of the 

 intestine where the typhlosole ends. Another small branch {a.r., fig. 79) goes to the 

 dorsal edge of the posterior hepatic orifice, probably supplying the gastric typhlosole. 



