The fifteen anterior somites, numbered from before backwards, the 'prostomial segment' counting as the first.
Plate XII. Earthworm, Lumbricus terrestris.
THE integument has been divided, except in the prostomium, down the middle dorsal line, and the greater part of the digestive tract has been removed, together with the pseud-haemal vessels, so as to show the nervous, nephridial, and reproductive organs.
a 1. Bilobed supra-oesophageal ganglionic mass; giving off from either outer angle a nerve which bifurcates very soon after its origin, and distributes itself in the prostomium.
a 2. Visceral or stomatogastric plexus of the right side. See p. an, ante. Described by Quatrefages, A. Sc. N. (3) viii. 1847, p. 36.
a 3. Commencement of ventral nerve-cord.
b. Pharynx, turned aside to the left, the right half of the organ, except the small portion upon which the right stomatogastric plexus, a 2, is seen, having been removed.
c1. First nephridium, or 'segmental organ,' opening externally in somite iv. Ordinarily the thickened muscular portion of the tube opens externally in the somite immediately posterior to that in which its internal funnel-shaped opening is situated. But as somite iv is not cut off from somite iii by a perfect dissepiment such as limits the somites after somite v inter se, the anterior funnel-shaped termination is not in a different somite from the one in which the coils of the gland are lodged.
c 2. Nephridium similarly modified to c 1.
c 3, c 4, c 5. Normal nephridia. The opening on to the exterior is usually close to the inner row of setae, though it may vary considerably, and even come to lie exteriorly and superiorly to the outer row of locomotor spines. The funnel-shaped internal opening is seen a short way from the outer edge of the nerve-cord, and near the ventral surface in the somite anterior to that in which the gland communicates with the exterior. In these organs the coils of the posterior part, which is much the larger, are connected by a mesentery-like lamina to each other and to the dissepiments of the somites.
d. Muscle passing up from one of the ventral muscles to attach itself to the capsule of the supra-oesophageal ganglion to which it stands in the relation of a powerful retractor.
e 1, e 2, e 3, e 4, e 5. 'Capsulogenous glands' of D'Udekem. These bodies are stated to be specially modified and greatly developed setiparous glands, which attain this prominence in the somites connected with the essential, and with the accessory organs of generation; amongst the latter of which the long inner setae of many somites may be reckoned, besides those here lettered € I to e 5. At e 2 we see a slip of muscle passing across the glandular mass, and connecting the inner with the outer row of setae.
f 1. Outer row of setae. Each seta is formed by a separate sac, and has a separate insertion. The setae of the aquatic Oligochaeta are, on the contrary, 'fasciculate' in their insertion. In the clitellum the outer row of setae may be enlarged to serve as copulatory organs.
f2. Inner row of setae. The spines are solitary in their insertion as in the outer row, but they are enlarged so as to serve as organs of adhesion in many somites, as, for example, in the tenth and fifteenth, as well as in the somites constituting the clitellum.
g1. Anterior receptaculum seminis of the left side, opening in the interval between the ninth and tenth somites, in the line of the outer row of setae.
g 2. Posterior receptaculum seminis, opening in the interval between the tenth and eleventh somites. These organs are very variable in size, according as they are full or empty.
h 1. Anterior vesicula seminalis of the left side. See p. 206, ante.
h 2. Middle vesicula seminalis of left side.
h 3. Posterior and largest vesicula seminalis.
i I, Internal funnel-shaped orifice of vas deferens anterior of right side.
i 2. Similar orifice of vas deferens posterior of right side. Nephridia are present in these somites, and are disposed in the ordinary manner. Diagrammatic: see pp. 206-8, ante, and Q. J. M. xx. p. 80-82, with woodcut, p. 79.
j 1. Vas deferens from anterior spermatic infundibulum of left side.
j 2. Vas deferens from posterior spermatic infundibulum of left side. The junction of the two vasa deferentia to form one common canal is well seen in the twelfth somite on the right side, and the ending of the common canal there formed is well shown on both sides in the fifteenth somite. The external opening of the common vas deferens has the shape of an oval slit, with its long axis transverse to that of the animal's body, guarded at the breeding season by prominent tumid lips. k 1. Anterior testis of right side. k 2. Posterior testis of right side.
/. The single ovary of the right side, occupying the same position relatively to the nerve-cord and the inner row of setae as the testes.
m. Infundibular ciliated mouth of oviduct.
n. Oviduct in the posterior of the two somites to which it is related. As it passes through the dissepiment separating the thirteenth from the fourteenth somite, the oviduct has a saccular dilatation - a receptaculum ovorum - ordinarily found to contain ova, appended to it. It then passes outwards in relation with the dissepiment, and opens externally to the internal row of setae. It is crossed just before its termination by nephridium, c 5, which is related to the same somites.
- The prostomium is numbered in this plate as the first somite. It appears, however, to be not a somite but an outgrowth of the somite numbered ii here. See p. 197, ante. The receptaculum ovorum is said, in Bergh's account, Z. A. ix. 1886, pp. 232-3, to belong to a septum - that which separates somite twelve from somite thirteen. It does so belong, provided the prostomium is not reckoned as a somite. But if it is reckoned as a somite, the septum in question is that which separates somite thirteen from somite fourteen. The usual mode of reckoning the somites followed in this plate has also been followed in the description of the preparations of the Earthworm, pp. 196-212, but it is probably Wrong.