NATURAL HISTORY OF THE SHIP-WORMS. 203 
of woody materials progresses, the cecum projects more and more posteriorly, and 
in specimens 2 mm. long (fig. 9) extends much beyond the posterior adductor muscle. 
The gills soon grow around the foot posteriorly, and in specimens 2 mm. long, 10 
to 12 days old, project much beyond the visceral mass (fig. 9). This same stage 
also shows the pericardial space, with its contained and associated structures, in 
the position which it occupies in the adult, distinctly posterior to the larger adductor. 
And in this, as in subsequent stages, the visceral ganglion lies at the posterior end 
of the pericardial cavity. 
There are a number of features in the organization and metamorphosis of the 
larva that seem to have a wider significance. One of these is the sudden and com- 
plete loss of the velum. Lovén thought that, in forms of lamellibranchs studied 
by him, it entered into the formation of the labial palps. These structures are 
present in the adult Y. gouldi only as the small ridges on the sides of a slight 
groove around the mouth; so that this form, in which they may be said to be absent, 
does not give evidence necessarily against the derivation of the palps of the adult 
from the velum of the larva in forms in which the palps are well developed. The 
velum in various lamellibranch larvee, however, is very much larger than the palps 
in early stages, so that most of it must be cast off or absorbed. In the newly 
attached oyster I have observed that the cells of the velum are absorbed more 
slowly, though the palps are developed somewhat later merely as ridges at the 
sides of the mouth. The evidence from X. gouldi and Ostrea virginiana, it seems 
to me, shows conclusively that the palps are not derived from the velum. The 
loss of the velum is an event not confined to the lamellibranchs. Wilson (1890) has 
observed that the trochal cells of Polygordius are suddenly cast off and eaten, as 
in XY. gouldi, and Pruvot has described the loss of the test in Dondersia. These 
all seem to be phases of one and the same phenomenon and indicate that the loss 
of a part of the ectodermal covering during metamorphosis in these and many 
other forms is a very primitive and general occurrence. 
The addition of the ship-worms to the forms which have heretofore been known 
to possess a byssus apparatus indicates that this structure is perhaps universal in 
lamellibranch larve, though in the adult it may become degenerate. I may add 
that in Ostrea virginiana a byssus apparatus is present in the newly attached larve, 
though here a secretion is thrown out for the attachment of the left valve and does 
not form a byssus thread. In forms like Yeredo and Ostrea the byssus serves for 
the attachment of the young bivalves, and apparently it has the same purpose in 
other forms in which it is present in the young (e. g., Pecten) but is lost in the adult. 
In Spheriwm and allied forms it serves to attach the viviparous larva to the wall 
of the brood chamber in the gill of the mother. All the known facts go to show 
that the byssus apparatus was developed to assist in the transformation of the free- 
swimming pelagic larva into the lamellibranch with an attached or other settled 
mode of life. Then, this transformation having taken place, the byssus may be lost, 
or it may be retained in forms which are permanently attached but lack other means 
than the byssus for attachment. 
The sheath of the crystalline style is well developed in the newly attached larva. 
Everything, however, indicates its derivation by transformation of the posterior 
end of the stomach. If we imagine the intestine leaving the blind end of the 
B. B. F. 1907—14 
