IGG U. S. p. R. R. EXP. AND SURVEYS ZOOLOGY — GENERAL REPORT. 



follicle of the ovary, whilst in reality the membranes may be stretched out or extended, and 

 the entire progeny loosened from ;ill adherence or connexion with tlieni. 



The male organs of generation consist of two spermaries, a right and a left, perfectly inde- 

 pendent from one another, having each its separate duct, discharging their contents into an 

 elongated cloaca, into which the bladder likewise empties its contents. This cloaca commu- 

 nicates with the exterior by a sub-circular opening, the edge of which being rather protruding. 

 Such is that apparatus : the same in its general structure as in the other osseous fishes. There 

 is no sheath enclosing the two spermaries, and this fact throws a considerable light upon the 

 morphology of the ovaries ; the latter being, in fact, two in number, but so closely connected 

 toL'^ether as to simulate a single organ. Thus the general disposition, not the plan of structure, 

 of these organs, is adapted to the mode of reproduction ; a single sheath being a more simple 

 adai)tation than two, one for each ovary. 



How the mechanical act of fecundation takes place we are not prepared to say from direct 

 observations : the eggs themselves must be fecundated witliin the ovarian sheath ; a copulation 

 of some sort is, therefore, required, and it is not improbable that at this period the eggs have 

 dropped from the ovarian membranes into the pouches or spaces between these membranes, in 

 which they are freely floating. 



There are a few points in the history of the development of the young which may be 

 deduced from partial observations represented on Plate XXVI. 



The hatching of the embryos takes place at an early period. After leaving the egg shell 

 they have an abdominal bag containing the remaining yelk which is to be gradually absorbed 

 duriu"- a period when neither the mouth nor the esophagus are formed. The fins are as yet 

 undeveloped (figs. 1 and 2). The eyes begin with a deposition of an external layer of black 

 pimentum, in every respect similar to the same organs in invertebrata (figs. 1, 3, and 5). The 

 head is anteriorly rounded, and the cleft of the mouth not yet apparent at the period when the 

 fins beo-in to develop (fig. 1). The caudal has the start over all the other fins ; its posterior 

 margin is first lanceolated, then rounded, with a convexity gradually diminishing until it is 

 perfectly straight, when a gradual emargiuation commences, and from a slight crescent reaches 

 the forked shape which it has in the adult. The soft and articulated portion of the dorsal and 

 anal fins next assume a development reaching extraordinary proportions, which they again 

 gradually loose so soon as free from parental sheltering. The posterior portion of these fins is 

 especially to be noticed as longer or deeper than tlie anterior portion (the reverse of what we 

 observe in the adult), and extending generally beyond the base of the caudal fin, a character, 

 therelbre not exclusively proper to the genus Rhacocheilua. The spinous portion of these same 

 fins on the other hand, develop but slowly and gradually, reaching their full growth at a late 

 period. The ventrals and pectorals are likewise tardy in their appearance. The scales are 

 fully developed belbre the young leave the ovarian slieath. 



The first notice that was made public in relation to these fishes appeared in November, 1853,^ 

 with additional remarks in May, 1854." New species of that family were likewise subsequently 

 described.' But the real knowledge of the remarkable peculiarities concerning some of their 

 habits was obtained in the spring of 1852 by Dr. Thomas H. Webb, while attached to the 



' Araer. Journ. of Sc. & Arts, secwid scries, vol. xvi, p. 380. 



^ Ibid., vol. xvii, p. 305. 



» Proc. Acad. Nat. Sc. Pbilad. VII, 1854, pp. 105, 122, 134, and 151. 



