Gross Anatomy

The reproductive system of the slug Athoracophorus bitentaculatus consists essentially of a gonad capable of producing eggs and sperm simultaneously, and a compound tubular passageway through which the sperm and eggs are conveyed externally. Three different major glands feed into this passageway; these are the albumen gland, which secretes a nutrient solution around each egg, the oviducal gland, which secretes a number of encapsulating mucus coats around each egg, and the prostate gland, which produces a secretion of unknown function in which the sperm are immersed. Portions of the passageway are lined with cilia, and the terminal portions are modified into copulatory apparatus. A cutaway diagram to show the relationship of the various structures is given in Figure 1.

The gonad consists of several lobes, closely fitted together. Each lobe consists of a number of tubular acini, each acinus being broadest at the surface of the gonad, and narrowing down to a small efferent ductule which communicates with the lumen of the hermaphrodite duct. The hermaphrodite duct is quite long, highly convoluted and of constant diameter along most of its length but narrow close to the albumen gland; it passes from the ovotestis downward and anteriorly to the medial surface of the albumen gland, which is compact, elongate, medially flattened and yellow in colour. The secretion of this gland passes into an albumen canal, embedded in gland tissue and lying close to the medial surface of the gland. The canal communicates directly with the hermaphrodite duct to form an irregularly shaped chamber, the fertilisation sac.

Two canals lead out of the fertilisation chamber. The first leads straight into the lumen of the most posterior portion of the oviducal gland. This extensive gland is folded into a number of broad convolutions with a continuous lumen. The second canal leading out of the fertilisation pocket is ciliated, and leads straight to the posterior portion of the prostate gland, where it opens into a narrow cleft which passes ventrally page 3

Fig. 1: Cutaway diagram of reproductive system, ventral view. Vertical component not to scale. The connection between the fertilisation pocket and the female duct is out of the plane of section, and is shown dotted.

Abbreviations: a. gl., albumen gland; f.p., fertilisation pocket; g., gonad; h. d., hermaphrodite duct; ov. gl., oviducal gland; p. gl. prostate gland; p.r.m., penis retractor muscle; pen., penis; pen. s., penis sheath; sp., spermatheca; u., uterus; vag., vagina; v. d., vas deferens.

page 4 through the prostate gland to communicate with the tubular sperm duct; this passes anteriorly to receive tributaries from other portions of the irregularly-shaped prostate gland. The sperm duct finally joins with the lumen of the oviducal gland to form a spermoviduct, which passes anteriorly to terminate in a narrow ventrally directed cleft. This cleft, at the bifurcatio spermoviducti, leads laterally into a muscular, convoluted uterus, and anteriorly into a narrow, highly convoluted vas deferens.

The lumen of the uterus carries extensive longitudinal ridges, which persist to the point at which the spermathecal duct enters the uterus; at this point the ridges become transverse. The spermatheca is moderately thin-walled, rounded, and attached to the anterior end of the uterus on the dorsal side by a short, tubular stalk, the opening of which is guarded by valves. Immediately anterior to the insertion of the spermathecal duct, the uterus makes a Y-shaped junction with the penis, laterally, and the vagina, anteriorly.

The penis of Athoracophorus bitentaculatus is basically a muscular convoluted cylinder, contained in a penis sheath, and narrowing distally to join the vas deferens. At the junction with the vas deferens, the penis sheath merges with the penis, and a penis retractor muscle is inserted. The origin of the penis retractor muscle lies in the body wall on the dorsal side posterior to the mantle area. The lumen of the penis is heavily ridged and papillated.

From its junction with the penis at the point of insertion of the penis retractor muscle, the vas deferens runs anteriorly over the surface of the penis to pass dorsally over the uterus down the anterior wall of the spermatheca, and then posteriorly to pass through the tissue of the anterior portion of the prostate gland to join the spermoviduct.

The vagina extends anteriorly from its junction with the penis and uterus to open at the genital orifice situated in the right margin of the head shield, just lateral to the right tentacle.