185 
Structure o/Amphibola avellana. 
tuous course forward, gradually narrowing, and then turns 
suddenly at a right angle to join the base of the penis. It is 
very muscular in structure, ciliated internally, and shows a 
well-marked peristaltic action. The penis (fig. 11, A) is short, 
thick, and bifid at the apex. At its base open two different 
organs : one, of a bright yellow colour, probably the prostate 
(fig. 11, «'), is a long coiled tube, which can be opened out, as 
shown in fig. 11. It is much like the prostate in Peronia. 
The other (fig. 11, A) is white, and formed of highly muscu¬ 
lar coiled tubes. It is probably a vesieula seminalis ; but I 
have never found any spermatozoa in it. The opening for the 
penis is situated just behind the right eye (fig. 1 , b). The 
retractor muscle is attached to the basal half. The sperma¬ 
tozoa are long and filiform, gradually enlarging anteriorly, 
but without a distinct head. The anterior portion is spirally 
twisted (fig. 13). They are active, and coil up on the appli¬ 
cation of fresh water. Their length is more than *01 inch, 
while their greatest thickness is about '00005 inch. 
The ova are elliptical, with an average length of '006 and 
an average breadth of *004 inch. After passing from the ovi¬ 
duct they are lodged on the exterior of the mantle, in a circu¬ 
lar patch, near the opening of the renal organ. I believe that 
they are impregnated here, as I have found what I took to be 
dead spermatozoa among them. Afterwards they acquire a 
thick coat, which gives them a bluish-white pearly appearance. 
The empty capsules have an irregular radiating structure and 
a circular hole at the top, through which the young animal 
has escaped (fig. 12). 
I have not attempted to make out the development of the 
ova; but the embryo does not appear to possess any velum, 
and I have never observed it to be externally ciliated at any 
period. 
EXPLANATION OF PLATE XXII. 
Fig. 1. Animal in its shell, a , respiratory aperture; b, opening of penis; 
c, eye at the base of the tentacles. 
Fig. 2. General view of the anatomy of the animal, a, oesophagus; b, 
gizzard ; c, stomach; d, intestine; e, rectum ; f, renal organ ; 
g, heart; h, columellar muscle; i, foot; k, operculum ; l, ovo- 
testis and liver; m, hermaphrodite duct; n, albumen-gland ; o, 
oviduct; p, vas deferens; q, penis. 
Fig. 3. Digestive apparatus, a, buccal mass (from above); b, extrinsic 
muscles; c, salivary glands; d, oesophagus ; e, crop ; f t caecum ; 
g, stomach; h, gizzard; i, liver; k, intestine ; l, rectum; m, 
anus; n, heart; o, aorta; p, auricle ; q, renal organ. 
Fig. 4. Buccal mass, viewed from the right side, a, mouth ; b, radula- 
sac ; c, extrinsic muscle; d, salivary gland. 
Fig. 5. Odontophore. 
Fig. G. A lateral tooth from the right side. 
