l86 BULLETIN OF THE BUREAU OF FISHERIES. 



The second makes a slight forward curve near its origin. From this point a somewhat 

 flattened forked process extends posteriorly, one prong of the fork passing on either 

 side of the third hfemal spine. These two prongs terminate in footlike enlargements in 

 the muscles of the anterior caudal segments. The third haemal spine makes a somewhat 

 stronger forward cur\'e near its origin than the second. From its proximal portion a 

 short, flattened, keel-shaped process extends posteriorly. The distal ends of these three 

 haemal spines are connected by a narrow band of cartilage. (PI. xvi, fig. 2, HS.) 



The interhaemals are correspondingly larger in the male than in the female and 

 are embedded in the large muscle controlling the modified anal fin. The one articulat- 

 ing with the third ray of the modified anal fin is greatly enlarged and articulates loosely 

 with the two anterior processes on which the muscle has its origin. (PI. xvi, fig. 2, IH.) 



The mechanism controlling the modified anal fin projects anteriorly into the abdom- 

 inal cavity to such an extent that the space allotted to the air bladder becomes somewhat 

 restricted. Consequently, the latter organ is relatively shorter and occupies a more 

 oblique position in the male than in the female. 



Ryder " (1885) has given us a brief description of the modified anal fin of Gambusia 

 and the mechanism by which it is controlled, which is in many respects erroneous. A 

 comparison of Ryder's description with the description given above will not be attempted 

 in this paper. The former description, published more than a quarter of a century ago, 

 was obviously not the result of an exhaustive study. 



Testis. — The testis, like the ovary, is a paired tubular organ and is not distinctly 

 divided. (PI. xvi, fig. 4.) It is located in the abdominal cavity dorsal to the posterior 

 portion of the intestine and just anterior to the large muscle controlling the anal fin. 

 The testis does not extend as far anteriorly as does the ovary, but, like the latter organ, 

 the left side of the testis is shorter than the right. 



The spermatozoa are contained in spermatophores, which are rounded or spherical 

 bodies, o.i to 0.2 millimeters in diameter. (PL xvi, fig. 5.) The walls of the spermato- 

 phores are exceedingly delicate. If the spermatophores are ruptured under the micro- 

 scope, the spermatozoa may be seen to escape freely even though they are still immature 

 and inactive. The spermatozoa are comparatively large. Each one is composed of a 

 comparatively large, elongated, slightly cur\'ed and bluntly pointed head, a middle 

 piece which is nearly as long but more slender than the head, and a long flagellate tail. 

 (PI. XVI, fig. 6.) 



In most of the spermatophores obser\-ed, the spermatozoa were inactive and 

 apparently curved around a small, bubble-like body, thus forming a more or less com- 

 plete ring. When the spermatophores were broken many of the spermatozoa were 

 released from this curved position and freed from the small, bubblelike body. The 

 tails, however, still retained a slight curve. The heads of the spermatozoa may be 

 readily obser\'ed in the spermatophores under moderately high magnification. They 

 are closely aggregated but show no regular arrangement. While no spermatophores 

 were observed in the genital organs of the female, it is highly probable that the sper- 

 matozoa are transmitted from the male to the female in these bodies. 



o Ryder, John A.: On the development of viviparous osseous fishes. Proceedings of the U. S. National Museum, vol. vin, 

 p. 143, 144- 



