S.A. NAT.. VOL. XtV. 
August 3 1st. 1933. 
136. ^ouih Just ''(ilidu Sh/'Hs. 
■attainment of maturit}% there is a supplementary lube built out 
from the edge of llie anal orifice. The shell contains a very slight 
organic basis, leaving no appreciable amount when dissolved in 
acid. It is formed of three distinct layers. The inner layer 
(hy]'>ostracum) composed of long prisms, roiincfed at the angles 
.and tapering at the ends. This layer is thinnest at the aperture, 
thickest towards the apex. The thick middle layer (ostraciim) 
built up of short prisms in bundles lying at right angles with 
-each other. 'I'he outer layer (periostracum) thin and wliolly 
sculptureless. It covers the sculpture, ribs, etc., of the shell. 
The radula is short, early rigid and curved, and incapable of be- 
ing used as a rasping organ as in other molluscs; its function is 
probably largely as a crushing plate, like the gizzard-plates of 
BuUaria. The formula of teeth is 1. 1. 1. 1. 1. (Pilsbry). The 
posterior aperture of the shell is emarginated by a ventral sinus, 
and is furnished interiorly with a dorsal and ventral valve, which 
are capable of being applied to one another. The Scaphopods 
seem to form an actual link connecting the bivalves with the 
univalves. In most points they agree with the bivalves; in only 
two respects — the shell and the radula — do they agree witli the 
univalves, and even in the matter of the shell they begin life as 
bivalves, for in the embryo this consists of two plates which later 
become united to form a tube. 
The shelled foraminifera found in the stomach of a Scaplio- 
pod are perfect, and the sarcode must be extracted from them 
by some secretion referable to the gastric juice of the Vertebrata. 
Eyes are absent as the)’ would be useless to an animal always 
buried in sand. There are no external organs of generation; but 
impregnation is effected by the male emitting the spermatozoa, 
and the female her ova at the same time, in the water. The pro- 
cess may be partl)^ compared to the chance shedding of pollen 
in the air by dioecious plants. Lacaze-Duthiers closely studied 
DentaHum vulgare (France) and noticed that the spermatozoa 
lived six hours after performing the act of fecundation. The 
egg is at first oval, afterwards pear shaped, and ultimately 
divided into segments like those of an Annelid. In the first 
stage of development the germ is motionless; in the second stage 
it is propelled by vibratile cilia, which are set round a large lobe 
in front, similar to that observable in the larvae of many mollusca, 
and it swims rapid!)’ ; in the third stage it crawls by means of a 
disc-like foot. In swimming it docs not come to the surface of 
the water, as do the fr)’ of the oyster and other mollusca. The 
shell is formed during the third period, but is only detected b)^ 
Its iridescent lustre, being exceedingly thin and transparent, a 
