In Lyntnceus stegnulis we have a curious exception to this mode of copulation, for in this animal the sexual organs are so placed that mutual impregnation is impossible, and accordingly fecundation is accomplished by a combination of individuals, each of which performs the office of a male to another, while to a third it acts the part of a female, and long strings of them are often seen thus united.
•Sexes distinct, that is, the ovigerous and impregnating organs placed in separate individuals.
This type of the reproductive apparatus extends through a wide range of animals, and is found in a great number of classes utterly dissimilar in outward form and internal struc ture; so that, in order to give a connected view of the comparative organization of the parts of generation, we shall be unavoidably com pelled to group together animals widely sepa rated by the laws of zoological arrangement. Feeling, however, that by so doing we shall lay before our readers a much more easily intelligible comparison of the organs belonging to our subject, we shall not scruple to bring together, in one view, analogous forms of the generative apparatus, in whatever classes they may be found. Animals in which the sexes are distinct may be divided into three classes ; the first including such as are oviparous, the second embracing the ovo-viviparous orders, while the third will comprehend the strictly viviparous animals. It will be seen that the terms here employed have been used from time immemorial, but nevertheless in a widely different sense to that in which the present state of our knowledge sanctions their application. To us it appears that we ought to regard all creatures as ovipa rous whose offspring, at the period of their escape from the ovum, are sufficiently mature to admit of their independent existence. In the ovoviviparous division, on the contrary, the ova are hatched and the embryo expelled at an early period of its formation ; the embryo is thus born in an extremely imperfect state, the materials for its future developement being supplied by the mammary secretion of the parent ; such is the case with all the marsupial orders. In the vivipara the earliest stages of growth are precisely similar to those which mark the progress of evolution in the ovi parous type, and the provisions made for the nourishment of the rudimentary being in every respect analogous ; the great distinction consists in the subsequent maturation of the embryo within a uterine cavity, and the forma tion of a placenta, which characterizes the highest form of mammifcrous animals.
The oviparous classes, which form by far the most numerous division, produce their young from ova, in which the germs of the future beings are developed for the most part subsequent to the expulsion of the egg from the body of the parent. In this case the ovum necessarily contains a sufficient store of nourishment for the support of the embryo during the whole period of foetal life, at the termination of which it is produced in a suffi ciently advanced stage of its growth to render it capable of independent existence. It will readily be perceived that under this division we include many animals which, according to the old meaning of the terms, were looked upon as ovo-viviparous or viviparous in their mode of reproduction ; a distinction which, as the words have been hitherto applied, appears to the writer by no means sufficiently grounded upon physiological views to admit of its conti nuance. It is certainly very true that some ani mals included in this division are found to pro duce their young in a living state ; but the mere hatching of the egg within the oviductus of the mother, instead of subsequent to its ex pulsion, is not a circumstance of sufficient importance to he regarded as constituting another type of the generative process, more especially as such an occurrence is entirely fortuitous, observation having proved that the same animal at one time produces its young alive and at another in the egg state, in obedience to circumstances connected with food, tem perature, or confinement. With this extension of the term, oviparous animals in which the sexes are distinct will be found in many classes belonging to the diploneurose, cyclogangliate, and vertebrate divisions of the animal kingdom, combined with modifications in the structure and arrangement of the generative apparatus, which it will be our business to trace.