NATURAL HISTORY OF THE SHIP-WORMS. 201 
ORGANIZATION AND LATE DEVELOPMENT. 
In their form and general organization the adult ship-worms are the most peculiar 
and striking of all the lamellibranchs. Their newly attached larva has the form 
and essentially the organization of the larva typical of most of the marine lamelli- 
branchs which are free-swimming during their larval development. The transfor- 
mation of the typical larva into the highly specialized ship-worm is so striking and 
takes place with such great rapidity as properly to be called a metamorphosis. 
THE LARVA. 
ORGANIZATION. 
The general shape of the newly attached larva (fig. 1) is that of a small clam, with 
equal shell valves. These latter are broader dorso-ventrally than long, so that this 
lamellibranch, which is more elongated as an adult, as a larva is more foreshortened, 
perhaps, than any other form. The youngest specimens I found were creeping 
over the surface of the wood by means of the very active tongue-shaped foot. 
The velum was in all cases retracted into the large velar cavity (vc), a large 
space in the anterior dorsal part of the shell cavity. This is best shown in figure 24, 
which represents a sagittal section of a newly attached larva. The foot (fig. 1, 7, 22 
at this stage is very long, ciliated over most of its extent, and angular at its posterior 
end. This angular portion is occupied by a well-developed though simple byssus 
apparatus which throws out a single simple byssus thread several times as long as 
the diameter of the larva. This serves to anchor the larva in the early stages of 
attachment, so that after the velum has been lost it may still return to its mooring 
if from any cause it lose its footing. 
The siphons are already well developed in the larva, the ventral inhalent or 
respiratory (is fig. 1, 7, 22) with ciliated sensory papille; the dorsal exhalent 
(es), a simple nonciliated tube. The gills have advanced but little beyond the 
stage figured by Hatschek (1880) for the viviparous larva studied by him. On each 
side of the body there are two large gill-slits, and in the gill membrane the rudi- 
ment of a third. The shipworm larva is a typical dimyarian. Both adductor 
muscles are present in their usual positions, the posterior (fig. 22, ap) already con- 
siderably larger than the anterior (aa). The posterior retractor muscles of the 
foot at this stage are attached in the umbonal region of the shell, just in front of 
the posterior adductor muscle. 
The alimentary canal is already highly specialized (fig. 7, 22, 23, 24). A com- 
paratively long esophagus (0e) leads into the stomach, from which a single compara- 
tively large liver vesicle projects as a diverticulum on each side. The sheath of 
the crystalline style (ss) projects from the posterior ventral portion of the stomach 
on the midline. The intestine leaves the stomach from the right side, anteriorly. 
The ecxcum of the stomach, peculiar to the Pholadacea, is present as a small hemi- 
spherical rudiment on the right side, just posterior to the opening of the intestine. 
The nervous system of the larva (fig. 7 and 59) is already highly specialized. At 
the sides of the wsophagus are the two cerebral ganglia, separated from each other 
