Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/723

Rh W K M 681 drilus, Eudrilus, Pcnchasta, &c.) there is an identical number of testes occupying the same segments. It is probable but not yet proved that in Uroch&ta and Moniligaster, where there is only a single seminal reservoir and vas defcrcns on each side, there is but one pair of testes ; at any rate, in Typhssus, where the seminal reservoirs and vasa deferentia are single, there is but one testis on each side placed in the 10th segment. Among the &quot;Limicolae&quot; there is rarely (Phreoryctes) more than a single pair of testes, which may be in the 5th (Naidomorpha), yth (Lumbriculidas), 10th (Tubifex), or llth (Enchytr&us, &c.) segment. (b) Seminal Ecservoirs. The spermatozoa are not, however, de veloped within the testes, but in special receptacles, the seminal reservoirs (seminal vesicles, vesiculte seminales). These structures frequently enclose the testes, with which they have been con founded by many writers. By the earlier writers the seminal reservoirs were regarded as ovaries; this opinion was due to the numerous parasitic Gregarines which these organs contain ; the encysted parasites were mistaken for ova. In Lumbricus terrcstris, &c. (Bergh), there is a single median reservoir, which encloses the testes, the funnels of the vasa deferentia, and the nerve cord in each of segments 9 and 10 ; with these are connected three pairs of lateral outgrowths situated respectively in segments 8, 10, and 11. In Allolobophorafcetida there are four pairs of isolated seminal reservoirs in segments 8, 9, 10, and 11; there is no median unpaired portion; and the testes as well as the vasa deferentia lie freely in the body cavity ; the first two pairs lie on the posterior septa of their segments, and open by an aperture into the cavity of the segment behind ; the last two pairs lie upon the anterior septa of their segments, and open into the cavity of the segment in front. In all cases the seminal reservoirs are formed as outgrowths of the septa. The cavity of the seminal reservoirs is broken up by anastomosing trabeculse, in the interstices of which the spermatozoa undergo their development. There is very great variety among earthworms in the number and arrangement of the seminal reservoirs, but there is no form known in which they are absent. In Urochasta, Typh&us, Titanus, and Diachseia there appear to be only a single pair, which are of great length. In Diach&ta (Benham) they occupy 20 segments. In many of the &quot; Limicolx, &quot; as was lirst proved by Lankcster in Tubifex, seminal reservoirs are found ; and where their develop ment has been traced they appear to originate as outgrowths of the septa. In some of the simpler forms, e. g., Chastogastcr, seminal reservoirs M, are not developed, but the testicular products float freely in the perivisceral cavity, where they undergo their de velopment. (c) Vasa Deferen tia. -It is a rule without any excep- vd- tions among the Oligochseta that the spermatozoa are car ried out of the body by special ducts, which perform only this function. In this respect the Olirjocliieta are in marked contrast to thePofr/C/tajto, where FIG. 4. Genital Segments of lAimbrifus (slightly altered the ripe spermatozoa from Howes, Biological Alias). The left side repre sents the immature, the right the mature condition, so far as the male reproductive organs are concerned. t, anterior pair of testes (the second pair are in the next segment); w, seminal vesicles; sp, spermathecre; rrf, vas deferens; o, ovary; od, oviduct; ro, recep- taculum ovorum; n, nephridia; nc, nerve cord. nc are either set free by a rupture of the body-wall, or are conveyed to the ex terior by means of the nephridia, which, however, may be slightly modified in relation to this function. These ducts are termed vasa deferentia ; they in variably open freely into the body cavity, and only sometimes (e.g., Lumbricus) acquire a secondary relation with the testes by way of the seminal reservoirs ; they are developed independently of the testes. In the Hirudinca and Platyhclminths, on the other hand, the efferent ducts are continuous with the testes, of which they appear to be mere outgrowths ; Nussbaum has, however, recently (Zool. Anzeigcr, viii. p. 181) stated that in Clcpsinc the vasa deferentia are developed independently of the testes. In their simplest form (in many Enchytrseidie) the vasa deferentia consist of a single pair of convoluted tubes, which open by a wide funnel-shaped aperture into one segment, while the external aper ture is situated in the following segment. Very generally in those Oligochasta the funnel-shaped expansion is continued into a wide cylindrical tube, which narrows abruptly on passing through the intersegmental septum into a slender tube ; the walls of both sections of the vas deferens arc formed of cylindrical ciliated cells ; the external aperture is often surrounded by several rows of large glandular cells. Among a large number of the lower Oliyochseta there is a single pair of vasa deferentia, which occupy in the same way two segments ; the internal funnel-shaped aperture opens into one segment, while the greater part of the tube and the external orifice are situated in the following segment. A further complica tion is, however, introduced in the form of a glandular terminal organ, which opens on to the exterior by one extremity, and com municates with the vas deferens at the other. In Stylaria lacustris this organ is pear-shaped, narrowing towards the external aperture ; it is lined by a layer of cylindrical glandular cells, outside of which is a layer of muscular fibres ; the whole organ is covered by large peritoneal cells of a glandular appearance. This organ is termed the atrium ; the vas deferens, which is short, only curved (not convoluted), opens by one end into the broad extremity of the atrium, and terminates in a ciliated funnel, which has this peculiarity that it does not lie in the segment in front, but in the same segment (in the 6th) as the atrium ; it is, however, closely applied to the intersegmental septum of segments 5-6. This condition of the vasa deferentia is exactly repeated in the earthworm Moniligaster bancelli ; the seminal reservoirs (Beddard) lie partly in the 8th and partly in the 9th segment ; the vas deferens, which is much coiled, lies in the same segment; it is con tinuous at one end with the seminal reservoir, and at the other with a glandular body opening between segments 9-10, which has a structure apparently identical with that of the atrium of Stylaria. Chastogastcr diaphanus (Vejdovsky, 15) has the same general dis position of the vasa deferentia, but the atrium is divided into a glandular &quot;vesicula seminalis,&quot; into which opens the vas deferens, and a distal non-glandular portion, which can be everted during copulation. The reproductive ducts of the Tubificidas are still further com plicated in their structure. In Tubifex rimdorum the structure of the atrium at an early stage is like that of Stylaria ; a simple globular or pear-shaped atrium is formed as an invagination of the integument ; later this is differentiated into two parts, as in Ch&toyaster ; an additional structure is, however, developed in FIG. 5. Male Genital Ducts of Various Oliyochxta. In most cases the whole length of the vasa deferentia is shown; the transverse lines indicate the boundaries of the segments through which they pass. The thick black lines indicate the atria, the dotted lines the muscular part of the atria. In F, G, II the penial seta? are shown lying in a sac opening in common with the atria. A, Moniliyafter liarwe/H (outside the atrium in A and C is a layer of glandular tissue); C, Pltreatothrix pragensis (after Vejdovsky); I), Eudrilus silricola; E, I ontodrilus marionis; F, Pericheeta armata; G, Typhxus gammii; H, Acanthodrilus none-ielamlix ; I, Ocnerodrilus (after Eisen). the form of a group of unicellular glands, collectively termed the prostate, which open into the glandular distal region of the atrium ; in the adult Tubifex the proximal section of the atrium is developed into a protrusible penis ; this is surrounded by a second invagina tion of the integument, which forms a penis sheath and part of the penis proper. The glandular part of the atrium (vesicula seminalis) is ciliated, but cilia are wanting in the penis. In Tclmatodrilus XX TV. 86