412 



ORGANS OF GENERATION. 



ova are retained until ripe for extrusion. 

 Near the tcrminaticD of this oviduct are placed 

 several additional appendages, some of which 

 are apparently destined to furnish an invest- 

 ment for the ova, whilst one, which is con- 

 stantly present, is probably a reservoir for the 

 seminal fluid required to fertilize the eegs 

 as they arc expelled. (See GASTEROPODA.)^ 



The external parts are so disposed that 

 during the copulation of two individuals the 

 male organ of each is introduced into the 

 orifice leading to the female apparatus of the 

 other, both thus impregnating and being im- 

 pregnated at the same time. 



In Lymntmi strtgintlis 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. 



FOURTH DIVISION. Sews distinct, that is, 

 the ovigerous and impregnating organs 

 placed in separate individuals. 

 This type of the reproductive apparatus 

 e xtends 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 stole 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 ovo-viviparous 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 mammiferouj 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 be 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. 



The earliest appearance of this type is found 

 in the cavitary Entozoa (Ccelelmintha), and the 

 sexual organs, both in the male and female 

 of these creatures, may be regarded as ex- 

 hibiting the greatest possible simplicity of 

 structure, consisting merely of secreting tubes, 

 which in one sex produce the seminal fluid, 

 in the other develope the ova. The seminal 

 organ, or testis of the male, is generally a 

 single tube of extreme length and tenuity, 

 winding in large folds around the alimentary 

 canal, and occupying a large portion of the 

 abdominal cavity ; when unravelled, its length 

 is found to be manv times that of the animal ; 

 at one extremity it dwindles down to a filament 

 of the utmost tenuity, which floats loosely 

 in the juices of the body, whilst at the op- 

 posite end it terminates in a prolonged tubular 

 penis, or organ of intromission, placed near 

 the anal orifice. In the females of some 

 species, as in Alcarit, the ovigerous system 

 is composed of two tubes, each exceeding in 

 length and tortuosity the seminal vessel of the 



