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On examining the male at the breeding season, the kidneys are seen to be considerably swollen, the enlargement being especially noticeable posteriorly (fig. 1 A). Sections of the kidneys reveal an altered condition of the sinuous tubules (fig. 1 A, b), the conical epithelial cells of which are swollen at their free ends and indefinite in outline. The nucleus of each cell is slightly displaced and occupies a more terminal position than in the normal condition. These epithelial cells are active in secreting the material used in constructing the nest. They perform the function, indeed, of cell glands, and their secretion is carried by the uriniferous tubes to the aiter ventral border of each kidney, where a large duct passes longitudinally. In cross section the ureters (pl. I, fig. 1 A, a, a) are oval, and their capacity is very great at this time, the walls being of dense fibrous tissue lined with pavement epithelium. Both ureters emerge from the renal mass near the posterior end, and descending in a forward direction become applied to the wall of the so-called urinary bladder, which at this point is somewhat attenuated, and passing anteriorly they open obliquely from without inward into the bladder. This structure, it is unnecessary to say, is not morphologically connected with the urinary receptacle of higher vertebrates, the lengthened course of the ureters, of which it is simply a dilated common portion, being due to its extraordinary development in the male stickleback. In a fish 51 inches in length it is about an inch long, and at its widest part one-fifth inch in diameter. Situated on the right side of the abdominal cavity. immediately below the swim bladder in the posthepatic region, it has the form of a capacious pyriform sac, ending blindly anteriorly, and diminishing in circumference as it passes backward (pl. I, fig. F, a). Before terminating posteriorly it describes a double curve, crossing over the intestine from the right to the left side (pl. I, fig. 6, b), and after a short parallel course passing on the ventral side of the intestine to the right side again (pl. I, fig. 6, c), debouching behind the genital pore (pl. I, fig. 6, d) into a urinogenital sinus, forming the posterior portion of a cloacal depression (pl. I, fig. 6, e), into which also the anus opeus (pl. I, fig. 6, f). The wall of the bladder consists of two layers, an internal epithelium (pl. I, fig. 1 B, a), which is readily detached, and a dense external connective layer (pl. I, fig. 1 B, b), which thins out as the blader enlarges anteriorly. Traces of an intermediate muscular layer appear posteriorly where the walls are extraordinarily thickened. The descending ureters (pl. I, fig. 1 B, C c) approach opposite sides of the bladder, that on the left proceeding obliquely below the common duct of the lasa deferentia, and passing forward and merging in the walls of the bladder on the left side.
This union is shown in the same transverse section which shows the union of the cas deferens of the left testis with that of the right. The course of the right ureter is shorter and more direct, as the bladder lies on that side of the abdominal cavity at this point. It coalesces with the right wall of the bladder precisely opposite the left ureter. As the bladder descends to cross the intestine inferiorly it twists, so that the left ureter is brought to the central side of the cervix of the bladder. Both return to the lateral position as the bladder crosses the intestine. The intestine now curves to the right, and the relations of the ureters become reversed, the right being below and the left rising to the dorsal side of the bladder. They increase rapidly in capacity, showing in cross section an extremely elliptical cavity, and as the bladder enlarges they pass obliquely into its chamber, thei: walls being continuous with the external layer of the bladder. Along this tortuous course the viscid secretion of the renal tubules reaches the bladder, where it is stored up. When first formed the secretion is simply a plastic jelly ; but a fibrillar structure appears to rapidly develop in it. Indeed this appearance is assumed while the secretion is contained in the ureters. The epithelial cells of the urinary canals exert so actively the secreting function that the bladder becomes much distended by the accumulating mucus, and at length it flows slowiy to the urinary aperture, where it emerges as a tenacious elastic thread which readily adheres to any external object on contact. It can hardly be doubted that this secretion can be extruded at pleasure, the walls of the bladder, assisted by the abdominal parietes, being sufficient to effect this; but it is produced so abundantly that it also often appears to ooze out involuntarily. Male fishes may often be seen with a glistening, pendulous, conoid mass hanging from the urinary aperture, and increasing in size until it becomes detached. Such Hask-shaped masses of mucus occur frequently in tanks where these fishes are confined and no opportunity is afforded for nest building. When, however, an appropriate mass of seaweeds has been selected by the male, the fish has merely to approach closely, so that the protruding mucus may adhere to a projecting frond, and by passing and repassing round the mass the weaving operation is accomplished. Occasionally ai rapid ejaculatory movement is observed, and it is interesting to note that the threads are not carelessly superposed, except when necessary for increasing the density of the nest, but are crossed at an angle by the varying movements of the fish, so that rhomboidal spaces are inclosed and a regular reticulum is thus produced (pl. i, fig. 5).
Often the tightly drawn thread snaps asunder, though its tenacity is extreme. The fibers then curl up and form a terminal pellet, many of which occur on the surface of the nest. As before remarked, the mucus is not merely a semisolid plasm, but assumes a funicular character while in the ureters. If one of the chords binding a nest together be examined, it will be found to consist of sereral strands, the cord itself measuring from 0.0016 to 0.0051 inch in diameter, and the constituent threads from 0.0008 to 0.00092 inch. These smaller threads again consist of fine homogeneous filaments, which adhere in parallel order. The parallel arrangement of the ultimate fibrils is very striking and quite (haracteristic (pl. I, fig. 4).
The stickleback is by no means a monogamist, as was once believed, but endeavors to induce a number of females to deposit eggs in the nest he has built. His bellicose tendency, always considerable, is now greatly intensified, for he is exceedingly jealous and takes offense at every appearance of intrusion or even approach to the domain which he has appropriated for his own. The males, too, are fewer than the females, and a consequence is that there are many furious battles. Smitt tells what may happen:
Two rivals rush with the speed of arrows against each other, deal a powerful side-stroke with their sharp ventral spines, and hasten with undiminished speed each back to his own domain. After a few onsets the superiority of the stronger combatant is demonstrated, his territory is extended, and he signalizes his triumph by a splendor of colours, while the vanquished lays aside his brilliant dress as though overcome by shame. While the males disport themselves in these chivalrous tournaments, or rather, fight for their nests, the females swim about in long troops of greater or less strength outside the battle ground, and now and then a male selects his temporary mate from the company. The female that heads the troop swims forward with rapid darts, followed by the others, suddenly stops and assumes a vertical position, with head turned toward the bottom. The others assemble round her and range themselves in the same manner, as densely packed as possible. When she has thus collected the troop round her, she suddenly deals a blow that scatters the whole crowd in an instant. This sport is often repeated, but the rapidity with wbich they disperse renders it impossible to observe whether it is always the same female that takes the lead or whether they change places. These dperations are continued as long as the sun is high in the heavens, for four to six days, according to the weather. It seems more than probable that during these evolutions the females lose some roe, which adheres to water plants, and that this is fertilized by the males that, perhaps only for the time being, have not built any nest for the eggs. Benecke has also ascertained that under certain circumstances, as, for example, when he finds a suitable crevice or secluded nook among the water plants-the male does not build any nest, properly so called. Thus we have to deal with two methods of spawning in which the eggs are developed where they fall, among the water plants, and the more connubial method in which the eggs are developed in a nest made by the male. But in any case the nest building is one of the most interesting parts of the life of the three-spined stickleback, and one which many have been in a position to observe.
One of the fullest and best considered, as well as earliest of the accounts of the habits of the stickleback was published in 1854 by Albany Hancock, in - Observations on the nidification of Gasterosteus aculeatus and Gasterosteus spinachia,” in the Transactions of the Tyneside Naturalists' Field Club for 18511854 (Vol. II, pp. 312-317). He provided an aquarium in May andInto this new home were put four Fig. 84.-Three-spined Stickleback male laying the
foundation of his nest. Aiter Coste. or five sticklebacks, [and] they at once made themselves perfectly at ease. One, without the least hesitation, took possession of a certain spot, which it guarded with the greatest pertinacity, attacking vigorously any of its companions that might happen to approach the chosen locality. The beetle, too, which sometimes came slowly paddling by, was pounced upon and unceremoniously tumbled over ; but secure within his scaly armor, as the knights of old, he little heeded the onslaught of his naked assailant, so overpowering all opposition he scrambled onward in his undeviating path.
This fish was rather small, had the throat of a bright red colour and the eyes of a brilliant bluish-green. At first, all the others were pale: but in the course of a few days one of them gradually assumed the rich hues of that just described, and soon afterwards it also became attached to a spot, taking up its abode in one of the corners of the trough. On examining attentively the two