The solitary Salpians derived from the aggregate generation are, on the contrary, produced by a process more complicated namely, by means of the sexual functions,—the concurrence of the eggs and the sperm.
Generation in the "aggregate" Salpians.
— With very few exceptions, the individual aggregate Salpians produce only one offspring throughout their lifet, so that, if we examine them at one period, anterior to fecundation, we find a single egg, and, at a later period, a foetus. The egg is distinctly visible within the young aggregate individual before it has left its parent (isolated) Salpian ; and as the fecundation of this egg takes place immediately, or at least a very short time after, the birth of the young aggregate Salpa, it can be examined before fecundation, that is, during the development of the aggregate within the isolated individual, or shortly after its birth. The egg is lodged in the thickness of the internal tunic of the mother, at a little dis tance from the inner wall of the respiratory cavity. It is very clearly distinguishable in the aggregate embryo at a period intermediate be tween the appearance of the solitary embryo as a bud and its full development. It is then situated above the visceral mass, at the an terior extremity of the body, and nearly in the middle line, raising the external tunic into a slight prominence. It is spherical, and consists of a vitellus, containing the germinal vesicle and spot, and invested with a membrane so thick, that M. Krohn is led to regard it as comparable with the " calyx" in birds. During the progress of the development of the ani mal, the position of the egg is altered, and it becomes situated on the side of the body, somewhat approaching towards its superior surface, and behind the second muscular band. This position is retained by the egg, and sub sequently by the foetus.
To the posterior extremity of the egg a cord is attached, which serves as a sort of peduncle, in general directed nearly horizontally back wards, and consisting apparently of a pro longation of the membrane that covers the vitellus. At the period when the egg oc cupies the anterior extremity of the embryo, this cord is proportionally thicker and shorter than at the subsequent periods of the em bryo's development. M. Krohn considered it as being the nutritive peduncle of the ovarian capsule, or membrane, enveloping the egg. This cord exists, as above described, up
to the time of fecundation, but it soon after wards disappears. The ovary, we see, is represented merely by the membrane that surrounds the egg, and which, as before re marked, may be compared to the calyx of birds. The testicle, on the contrary, is well developed, hut, not increasing in hulk in proportion to the growth of the young Salpa, it only acquires its greatest development in nearly adult animals. It is always found in the neighbourhood of the intestine, but its position varies in different species. Some times occupying the centre of the visceral mass or nucleus, formed by the intestinal loop and its appendages, its presence is evident only in proportion as it raises those parts ; sometimes showing- itself quite exposed, it more or less covers a large portion of the digestive apparatus. This organ in the aggregate form of Salpa pinnata, S. pro boscidalis, &c. is spindle-shaped, and rests against the intestine, and has been taken for the liver by Cuvier, Chamisso, and Meyen. The testicle is composed of a greater or less number of ramified canals, the last ramifica tions of which end in culs-de-sac. All these canals end in a principal duct, which, passing along the terminal portion of the intestine, opens at the side of the anus in the great natatory or respiratory cavity traversing the body of the Salpa.
The testicle is very much slower in its de velopment than the egg organ in these ani mals ; and, as young aggregate Salpians are met with in which a fecundated egg and undeveloped testicle are co-existent, these individuals must have had connection with others in which the development of the latter organ was further advanced ; the sperm that fecundated the eggs being supplied by another group of animals. The maturity of the sperm does not coincide with that of the egg, yet, as the two sexual organs are present in the same body, these animals are herma phrodite, although probably not self-impreg nating. They have two generative functions to perform : the one to produce a new being, the other to fecundate an ulterior genera tion of animals similar in all respects to themselves. The "aggregate" Salpians pro bably perish soon after they have given birth to their "isolate" offspring.