‘NATURAL HISTORY OF THE SHIP-WORMS. 211 
the free portion of the growing point has become angular, and at the sides of the 
angle the two series of perforations are formed progressively (fig. 3), the outer always 
slightly in advance Of the inner. The corresponding slits of the two series push in 
(in the direction of the lower arrows in fig. 37) till they meet each other and till they 
push through to the epibranchial cavity. These in-pushings divide the original 
blood space of the growing point into flat spaces separated from each other except 
at two points, the openings into the afferent and efferent branchial veins. The 
median portion of the original blood space remains undivided as the afferent vein, 
and by the disappearance of the median part of the walls of the two growing points 
as they fuse together, the afferent veins of the two sides unite, posterior to the 
visceral mass, to form the single large, median, afferent branchial vein. The 
undivided outer dorsal portion persists as the efferent branchial vein on each side. 
The walls of adjacent slits are connected by numerous connective-tissue cells (fig. 
38) so as to form the gill laminz, the name given to them by Quatrefages and more 
appropriate for the gill elements in ship-worms which (except the anterior eleven) do 
not form filaments. From the mode of formation it is seen that there is a large 
flat blood space in each lamina and that there is a free flow of blood through the 
lamina (in the direction of the arrows, fig. 37) between the afferent and efferent 
branchial veins. 
Ina young ship-worm a half centimeter in length (somewhat later than the stage 
represented in fig. 3) there is on each side a continuous series of seventy-five or more 
gill filaments (filaments and lamine), stretching from the mouth region around the 
sides of the body and posterior to the visceral mass. Soon afterwards the ‘‘fila- 
ment” between the tenth and eleventh (usually) gill-slits broadens from before back- 
ward. This growth increases till, in large specimens of the adult, the anterior 
eleven filaments (it may rarely be ten or twelve) are separated from the rest of 
the gill by a space of 10 cm. or more. They retain the structure, however, and 
doubtless the function, of gill elements, though in the adult they form a series of 
simple bars attached at both ends on the sides of the ‘‘head”’ (fig. 6). In reality 
the first and eleventh filaments are but half filaments. 
In ship-worms the epibranchial cavity forms a single long canal posteriorly (fig. 
10, 31, 32), but is divided anteriorly where the gills of the two sides diverge from 
each other. As the anterior eleven filaments become separated from the rest of the 
gill, the epibranchial cavity between these two parts of the gill becomes a long, 
very narrow canal (ep ca, fig. 26-29), which lies in the mantle on each side, external 
to the afferent branchial vein and adjacent to the groove described below. 
The two limbs of a gill lamina (fig. 37) form almost a right angle with each 
other. At the angle there is a ciliated groove (fig. 30-32, 37) which extends the full 
length of the gill in young specimens (fig. 3), and in adults, in addition, connects 
the anterior eleven filaments with the rest of the gill (fig. 6, 26-29). In the adult the 
connecting part of the groove, then, is really a part of the gill and is homologous 
with the groove of one filament in other parts of the gill. The minute structure of 
the groove is as follows: The lining cells are in the main strongly ciliated and col- 
umnar (fig. 44), but there are distributed among them numerous mucous gland cells. 
As already noted, the internal surface of the mantle opposite the edge of the gill 
