ON THE BRITISH ANNELIDA. 265 



In order to facilitate the comprehension of the succeeding description, it 

 is proposed to enumerate the component parts of the sexual apparatus of 

 Nais in the order of their position from before backwards. 



The two pear-shaped sacculi (a, a, Plate VIII. fig. 72) are sufficiently promi- 

 nent and defined in outline to be traced satisfactorily by the eye. The fundus 

 of these sacculi exhibits no traceable orifice of communication with the glandu- 

 lar bodies (c, c, c', c') by which they are enreloped ; it is quite certain, however, 

 that this communication exists. The interior of these sacculi is not lined by 

 vibratile epithelium ; they lodge a peculiar, flexible, arrow-headed organ (b,b), 

 which is often seen extruded through the orifice (a') to a considerable di- 

 stance ; this organ is an indisputable intromittent instrument. Its axis is 

 hollow, and its root is structurally identified with the parietes of the vesicle 

 in which it is contained. The large-celled glandular masses (c', c) are tes- 

 ticular, and contribute the fluid which is emitted by the penis (b') through 

 the orifice (a') into the vulva (d), if not of the same, of another individual. 

 No other parts than these masses (c', c), discoverable in Nais, are charged 

 with sperm-cells, or, as Dr. Farre has designated them, ciliated corpuscles 

 (»»). These organs, therefore, are evidently the sole male apparatus of JVais ; 

 they furnish the secretion which, when introduced into the uteri (c?'), fecun- 

 date the ova contained in the large masses (g). These last masses are ulti- 

 mately composed of the elements (n), which consist of vitelline, nucleated, 

 orbicular cells. Through a channel, not yet clearly defined, these ova find 

 their way into the convoluted duct (/), which is prolonged from the fundus 

 of the uterus (d), and terminates in a remarkable fimbriated extremity (e), 

 through which the fertilized ova escape into the open cavity of the perito- 

 neum. This singular organ, which may be divided into the uterine ca- 

 vity, the fallopian duct, and inorsus diaholi, is repeated in every segment, 

 as seen at i, k, &c. In the posterior units, however, of this system the 

 large ovarian masses (g) are not reproduced; they are peculiar to the first 

 and most anterior of the system. In the posterior they are replaced by an- 

 other description of stroma tons tissue (as seen a,tj,J), which embraces the 

 mid-portion of each duct. In the absence of any demonstrable male appa- 

 ratus in connexion with these posterior utero-ovarian organs, there remains 

 no alternative but to believe that the fimbriated extremities (k, k)JIoating 

 freely and loosely in the fluid of the peritoneal cavity, and lined internally 

 with active cilia, take up sperm-cells from the fluid of the peritoneal cavity, 

 in which they may be seen suspended, conveying them through the duct 

 until they reach that portion (^j) which is embraced by the ovarian tissue. 

 At this stage the ova from this tissue are detached into the duct, where they 

 come into direct contact with the spermatic elements. The ova thus ferti- 

 lized travel onwards under ciliary agency, for which these parts are quite 

 remarkable, and finally escape at the outer orifice (?', i) ; they escape as ova. 

 Nais is therefore an oviparous worm. Young worms are never found in any 

 part of the body, nor at any time have true ova been seen in the fluid of the 

 peritoneal cavity. 



Nais is the only worm jet known in which the sexual organs commu- 

 nicate thus directly with the peritoneal cavity, and in which the fluid contents 

 of this cavity enacts a mechanical part in propagating throughout the female 

 elements of the reproductive system the fecundating fluid. It is not, how- 

 ever, the only Annelid in which the female moiety only of the sexual system 

 is segmentally repeated. In the Earth-worm, the testicular masses were 

 shown to be circumscribed to one region of the body, while the utero-ovaria 

 were reproduced in every segment. The Terebellce also exhibit the tendency 

 to repetition only in the female elements of this system. The male glands 



