This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(589). Commencing with the male organs, we are not surprised to find the testes divided into numerous distinct masses, or, rather, repeated again and again, in conformity with a law to which we have already alluded. The glands that apparently secrete the seminal fluid are about eighteen in number (fig. 110, e, f), arranged in pairs upon the floor of the visceral cavity. Along the external edge of each series there runs a common canal, or vas deferens, which receives the secretion furnished by all the testicular masses placed upon the same side of the mesial line, and conveys it to a receptacle (d), where it accumulates. The two reservoirs, or vesiculoe seminales (dd), if we may so call them, communicate with a muscular bulb (c) situated at the root of the penis.
* See Brandt und Ratzeburg, Med. Zool. tab. 29. 1 Ann. d. Sci. Nat. vol. xxii.
The penis itself (a) is frequently found protruded from the body after death; it is a slender tubular filament, which communicates by its origin with the contractile bulb (c), and, when retracted, is lodged in a muscular sheath (b.) The male apparatus is thus complete in all its parts: the fecundating secretion derived from the double row of testes is collected by the two vasa deferentia and lodged in the receptacles (d d); it is thence conveyed into the muscular cavity (c) situated at the root of the male organ of excitement, through which it is ultimately ejected.
(590). The ovigerous or female sexual organs of the Leech are more simple in their structure than those that constitute the male system. They open externally by a small orifice situated immediately behind the aperture from which the penis is protruded, the two openings being separated by the intervention of about five of the ventral rings of the body. The vulva, or external canal, leads into a pear-shaped membranous bag (fig. 110, g), which is usually, but improperly, named the uterus. Appended to the bottom of this organ is a convoluted canal (h), that communicates with two round whitish bodies; these are the ovaria. The germs which are formed in the ovarian corpuscles, therefore, escape through the tortuous duct (h) into the uterus (g), where they are detained for some time, prior to their ultimate expulsion from the body. The exact nature of the uterine sacculus, as it is called, is imperfectly understood: some regard it as a mere receptacle wherein the seminal fluid of the male is received and retained until the ova come in contact with it as they pass out of the body, and thus are subjected to its vivifying influence; other physiologists believe that the germs escape from the ovaria in a very immature condition, and suppose that during their sojourn in this cavity they attain to more complete development before they are ripe for exclusion; while some writers go so far as to assert that leeches are strictly viviparous, inasmuch as living young have been detected in the interior of this viscus: but all these suppositions are easily reconcilable with each other.
There is no doubt that the seminal liquor is deposited in this reservoir during the copulation of two individuals; neither would any one dispute that the ova are collected in the same cavity before they are expelled from the body. As to the discussion whether the young are born alive or not, or, as it is generally expressed, whether leeches are oviparous or viviparous, it is in this case merely a question of words; for, in a physiological point of view, it cannot make the slightest difference whether the ova are expelled as such, or whether, owing to their being retained by accidental circumstances until they are hatched internally, the young leeches make their appearance in a living state.
Fig. 110. Generative apparatus of the Leech.
(591). Leeches are oviparous. The ova remain in the uterus for some time, where they become invested, first with a serous membrane, and then with a glutinous fluid which remains attached to them after their expulsion, and serves as a protecting covering after they are deposited in the clay and holes of the sides of ponds. They generally deposit these cocoons from May to the end of September.
Such was the generally received view of the arrangement of the generative organs of these Annelidans, as given in the first edition of this work. We will now proceed to describe the reproductive system of the Leech, as deciphered by Dr. Williams in his most recently published version*: -
(592). The segmental organs of the Leech tribe exist under very readily demonstrated conditions. After pinning the common leech down carefully to the trough, and opening the body by a longitudinal incision along the dorsal aspect, the whole stomach and its diverticula must be minutely picked away. The dissection should then be washed with very gentle streams of fresh cold water, in order to remove the blood, which obscures everything. The object being thus carefully cleansed, and then floated in water, a full view of the segmental ovarian and the median testicular systems will present itself.
(593). The one consists of a bilateral series of extremely delicate, floating, pearly-looking membranous organs, equalling in the number of their pairs that of the annuli marked upon the integument. The other, more medianly situated and bilateral, also consists of two series (one on either side) of little spherical white bodies, tied together by an intermediate thread, which unite at a common point anteriorly. A third element should be noticed, namely the small sacculus, which, immediately behind the anterior mass* of the united testes, lies also on the median line.
(594). According to Dr. Williams, every segmentary organ in the body of the Leech is not only an excretionary instrument for the discharge of the cavitary fluid, but is also an ovary. The tube which proceeds from one limb of the organ and terminates in a spherical membranous vesicle is the counterpart of the ciliated extremity of the segmental organs of Nat's and Lumbricus. Both open into the cavity of the body. In the Leech, however, neither the vesicle, nor the tube into which it is prolonged, is ciliated, but, in lieu, they are highly contractile. The vesicle is the respiratory sacculus of Duges (§ 581.) It does not, as Duges contends, communicate externally by an orifice in the integument, but internally. The tube which supports this vesicle arises from that limb of the segmental organ which contains the ova in their least-developed condition. It is probable therefore that the chylaqueous fluid which this tube is designed to discharge flows throughout the entire extent of the organ, from the point of entrance to that of exit, before it escapes externally.