HABITS, MORPHOLOGY, AND EMBRYOLOGY OF GAMBUSIA AFFINIS. 1 85 



young fish, if, as is usually the case, it comes out head foremost, may be readily explained 

 by the tapering form of its body and by its own swimming movements. That some force 

 is necessary, however, for the extrusion of the young is evidenced by the perceptible 

 contractions of the muscles of the abdominal walls of the parent just before the young 

 is extruded. 



MALE. 



Modified anal fin. — The male members of the species may be readily recognized by 

 the modified anal fin, which functions as an intromittent organ. The third, fourth, 

 and fifth rays of this fin are enlarged, greatly elongated, and variously curved. All of 

 the rays are composed of segments. The diameter of each segment is slightly greater 

 at the ends than in the middle. Thus each ray shows a series of slight circular ridges. 

 These ridges are most prominent on the third ray, which is the largest of the elongated 

 rays and has a slight backward curve near its proximal end. The distal portion of this 

 ray bears a row of short, pointed spines on its anterior aspect, while posteriorly it is 

 fringed, a short distance from the tip, by a dentate ridge apparently in the fin membrane. 



The proximal portion of the fourth ray has a gentle forward slope until it comes 

 into close proximity with the third. From this point the former ray extends distally 

 parallel with the latter. The fourth ray is slightly longer than the third. Its distal 

 portion is divided, the two divisions diverging for a short distance and again coming 

 in contact with each other at the tip. The anterior division bears a few very small 

 spines anteriorly. The posterior division bears a considerable number of short, slender 

 spines posteriorly a short distance from the tip. The proximal ones of these spines are 

 arranged in two groups of three spines each. The fifth ray makes a short, sigmoid 

 flexure at its proximal end and then extends distally parallel with the fourth. Near its 

 distal end it makes another slight sigmoid flexure and terminates in a small hammer- 

 shaped enlargement which interlocks with a slightly recurved hook on the posterior 

 division of the distal portion of the fourth ray. The third, fourth, and fifth rays of the 

 anal fin are bound together by the fin membrane. The fifth ray may be brought for- 

 ward at one side of the fourth until it comes into close or immediate proximity with the 

 third. In this manner a groove or tube is formed through which the milt is transmitted 

 into the genital aperture of the female. The first two and the last five rays of the anal 

 fin are somewhat modified but not elongated. 



The rays of the modified anal fin are illustrated in figure 2, plate xvi. Figure 3, 

 plate XVI, shows the distal portion of the three elongated rays drawn in detail under 

 higher magnification. 



Mechanism controlling anal fin. — The modified anal fin is controlled by a powerful 

 muscle which is inserted on the proximal ends of the rays of the anal fin and has its 

 origin on the modified haemal spines of the first three caudal vertebras and a similar 

 process projecting vent rally from the fourth to the last abdominal vertebra. This 

 muscle stands in an almost vertical position and is so large that it causes a perceptible 

 bulging of the body walls just above the vent. (PI. xvi, fig. i.) 



The process projecting ventrally from the fourth to the last abdominal vertebra 

 has a slight forward slope. The modified haemal spines of the first three caudal ver- 

 tebrae project forward into the abdominal cavity in an almost horizontal position. The 

 first haemal spine is nearly straight, having a slight downward cur\'e near its distal end. 



