A lth ough several distinguished com parative anatom ists have done m uch by th eir labours to elucidate th e general organization of th e A nnelids, the reproductive system of organs in this class has been little, if a t all, investigated. O n this account th e author of this memoir is tem pted to lay before th e Royal Society th e results of a systematic series of researches on this subject, on w hich he has been, for some years, laboriously occupied. These researches conveniently arrange

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... selves under two leading divisions. The first will include a m erely anatom ical description of the chief or typical varieties of form and structure w hich these organs are found to exhibit in the leading genera of the class; while th e second will em brace a b rief statem ent of such homological views as the ascertained facts w ith respect to th eir structure, form, and anatom ical relations may appear to w arrant. As the organ upon w hich always, in some p a rt or other of th e body of the Annelid, the office of reproduction devolves, is frequently in other parts diverted to other p u r poses, and modified in outw ard characters, it will prevent circuitousness of expression if at th e outset a nam e be adopted, under which, w hatever its place, size or form, this organ may be generically distinguished. U nder th e appellation of th e " segm ental organ*," accordingly, it is proposed to de scribe th a t viscus upon the basis of w hich, under several striking variations of place and figure, are always ingrafted th e tru e generative structures. By way of a general historical introduction, the au thor will content him self w ith th e following citation from the recently published lectures f of Mr. H u x l e y , exhibiting th e state of knowledge on this subject at the date of its publication, Ju ly 23, 1856. " T he genitalia of th e typical A nnelida are excessively simple in their stru ctu re; indeed, special reproductive o * This is an unobjectionable t it le ; it implies no theory, it simply states the fact that the organ is repeated more or less regularly in the segments of the body. N o other organ in the body of the Annelid is seg mental;" every other organ is continuous throughout the body. This is the case with the alimentary, the nervous and the vascular systems. The feet are not " organs." Indeed, so intimate in an anatomical sense is the relation between the segmental organ and the foot, that in the development of the embryo Annelid they constitute one system. The hollow bases of the feet are so frequently the depositories of the genera tive products, that they may be regarded as bearing the same relation to the segmental organ as the uterus does to the ovaries of the Mammal. t Medical Times and Gazette, 1856-57. MDCCCLVIII. O be said to exist in most, the generative products being merely develope the walls of the perivisceral cavity into w hich they eventually freely float, m aking thenway out in a m anner which is n o t quite understood a t p re s e n t; probably throu g h some tem porary or perm anent apertures a t th e bases of th e parapodia." From this passage it is obvious th a t by th e m ost recent expounders of th e science of comparative anatomy, it is form ally asserted th a t in the A nnelida " special reproductive organs can hardly be said to exist." I t will afterw ards appear th a t the facts to be related in this m em oir conduct th e anatom ist to a widely different conclusion. A brief and special historical introduction will be prefixed to each departm ent of th e subject. By this arrangem ent references to th e researches o f oth er observers will be m uch facilitated. The families of A nnelids selected for exam ination w ill be placed in juxtaposition, more on account of an affinity o f type presented by th e " segm ental organ " th an in deference to any oth er principle of classification. T he Naidea, L um bricidea, H irudinea, Terebellidse, Nereidse, &c. will on this ground be taken in the order in w hich they are nam ed. Naidea.-I n his R eport* on th e A nnelids, th e au th o r m ade th e following statem ent with reference to the segm ental organ in th e genus N a is : " T he whole reproductive system is lim ited by D u g e s to th e glandular mass w hich is so readily observed about the anterior th ird of th e body, whereas in reality this only constitutes one segmental , more developed only th an those w hich are repeated in every ring of the body." A lthough in th e present m em oir th e author will avail him self of th e opportunity to cor rect certain inaccuracies com m itted in his " R eport " w ith respect to th e interpretation there given in relation to th e segm ental organ in N ais, he will only a t present observe, th a t in 1852 he drew atten tio n to th e fact th a t th e generative structures were developed upon one, two or m ore of th e segm ental organs common to alm ost every ring of th e body. To M. D u g e s th e segm ental organs, as distributed throughout th e two posterior thirds of th e body, were unknow n. No special description of them has been given by any subsequent anatom ist. I t is doubtful w hether the genus N ais is included by L e yd ig in his recent m em oir *{% " U eber den B au und die systematische Stellung der Raderthiere," while incidentally referring to th e fam ily of th e Lum bricidea. W h eth er this be the case or not, the au th o r is not acquainted w ith any special account of this organ in this group of Annelids. T he following description is draw n entirely from his own researches:-I n the freshw ater Naides the segm ental organ is readily examined. A few individuals should be selected and placed betw een two slips of glass, slightly compressed, and thus viewed as transparent objects. || This is a minute terrestrial Lv/nibricus. It is found in July and August in the fine loamy earth of gar dens. It is n o t the young of the common Earth-worm. 100 DR. T. WILLIAMS'S RESEARCHES ON THE STRUCTURE AND HOMOLOGY * Sran<l edition of the Hegne A n i m a l , the female system in Ilirudo va is figured and described as consisting of a single pouch, the uterus (with its appended vagina) and two 116