NATURAL HISTORY OF THE SHIP-WORMS. 215 
to lie on the morphological ventral side of the intestine, though apparently on its 
dorsal side, and the relations of the various parts of the circulatory system to each 
other and of the system as a whole to associated parts have been radically changed. 
The youngest stage of the circulatory system I have observed in detail is in 
specimens 2 mm. long, in which the heart consists of two almost completely sepa- 
rated halves (fig. 52). On each side a more or less spherical auricle (aw) lies lateral 
and slightly ventral to, and leads into, a more or less spherical half of the ventricle 
(ve). Each half of the ventricle sends a very narrow, vessel-like portion toward 
the midline, where the two sides unite. In this middle portion there are two semi- 
lunar valves (fig. 55) on the dorsal and ventral sides, and from this point two vessels 
emerge. One runs anteriorly, and, bending around the posterior adductor muscle, 
is continued posteriorly in the mantle. At this stage the visceral mass has projected 
but little posteriorly (fig. 3), and the second vessel from the heart, somewhat smaller 
than the other, runs ventrally into the visceral mass. These structures are shown 
in section in figure 54, which is of a longitudinal section through the median part 
of the ventricles and aorte in a specimen 4 mm. long. 
In the stage in which the heart is developing the stomach and cecum already 
occupy most of the visceral mass and the gills are very wide apart. This may 
account for the wide separation of the two halves of the heart. In development 
posteriorly the gills advance ahead of the other structures and, accompanying 
them, the two sides of the heart are drawn out backward so as to lie side by side. 
In the adult (fig. 53) the two halves of the ventricle (ve) have fused on the midline, 
except at the posterior end, where the two sides still project as somewhat hemi- 
spherical masses. Internally, however, the lumen remains divided (fig. 29) through 
half of the extent of the ventricles. At the anterior end the ventricle has the 
shape of an elongated cone. The two auricles accompany the gills in the posterior 
development of the latter and come to lie side by side like two large vessels in the 
posterior half of the pericardial cavity. Each projects into the ventricle on its 
own side and valves separate the cavities of the auricles from that of the ventricle 
(fig. 53). 
The pericardial cavity of the ship-worms (fig. 10, 29, 30) lies on the apparent 
dorsal, but really on the morphological ventral, side of the visceral mass. It is 
very large, extending from the posterior adductor to the visceral ganglion through 
a quarter of the length of the animal. In NXylotrya gouldi it narrows in front 
to form a canal which projects beyond the wider part to the posterior adductor 
muscle. About two-thirds of the distance from the visceral ganglion to the pos- 
terior adductor (fig. 10) the anterior end of the ventricle dips down through the 
pericardium into the visceral mass. This point is the end of the ventricle and the 
beginning of the aortz, the end of the ventricle being marked off by two semilunar 
valves which project forward on its dorsal and ventral sides (val, fig. 55). From 
the end of the ventricle two vessels are given off. The larger (avp, fig. 10, 56) 
runs forward (fig. 26-28) in the visceral mass and passes ventral to and in front of 
the posterior adductor, to bend over the latter and enter the mantle as the large 
dorsal or posterior pallial artery. This runs posteriorly as a single vessel in XY ylotrya, 
at the right side of the anal canal and epibranchial cavity (fig. 26-32, da) to the 
posterior end of the body, where it divides into the two paired arteries of the siphons. 
