Didelphia, the alternative name of the group was given in allusion to the circumstance that the uterus has two separate openings; while other features are the inclusion of the openings of the alimentary canal and the urino-genital sinus in a common sphincter muscle, and the position of the scrotum in advance of the penis.
P. Hill, "Contributions to the Morphology of the Female Urino-genital Organs in Marsupialia," Proc. Linn.
From them are developed two distinct types of histological elements; the genital cells and the cnidoblasts or mothercells of the nematocysts.
A third point of dispute is whether the nematocysts ar:e formed in situ, or whether the cnidoblasts migrate with them to the region where they are most needed; the fact that in Hydra, for example, there are no interstitial cells in the tentacles, where nematocysts are very abundant, is certainly in favour of the view that the cnidoblasts migrate on to the tentacles from the body, and that like the genital cells the cnidoblasts are wandering cells.
The gonotheca is formed in its early stage in the same way as the hydrotheca, but the remains of the hydranth persists as an operculum closing the capsule, to be withdrawn when the medusae or genital products are set free (fig.
Ge, Genital glands.
Free in a mature condition as the so-called " genital swimming bells," comparable to the Globiceps of Pennaria.
Cloaca and Genital Organs.
The next chamber, the urodaeum, is small, and receives in its dorso-lateral wall the ureters and the genital ducts; above and below this chamber is closed by circular folds, the lower of which, towards the ventral side, passes into the coating of the copulatory organ when such is present.
In male beetles, however, the two pairs of genital processes (paramera) belonging to the ninth abdominal segment are always present, though sometimes reduced.
The morphology of the abdomen, ovipositor and genital armature is dealt with by K.
It is therefore by no means certain that so profound a difference embryologically can be asserted to exist between the excretory nephridia and the ducts leading from the coelom to the exterior, which are usually associated with the extrusion of the genital products among the Chaetopoda.
B, Diagram of the nephridium of Alciope, into which opens the large genital funnel (coelo mostome).
It has been shown that in Tubifex, and some other aquatic Oligochaeta, the genital segments are at first provided with nephridia, and that these disappear on the appearance of the generative ducts, which are coelomoducts.
Beddard) the funnels of the pronephridia disappear except in the genital segments, where they seem to be actually converted into the genital funnels.
At the least there is no doubt that the genital funnels are developed precisely where the nephridial funnels formerly existed.
If the genital funnels are not wholly or partly formed out of the nephridial funnels they have replaced them.
In the genital segments of Eudrilus the nephridia are present, but the funnels have not been found though they are obvious in other segments.
Here also the genital funnels have either replaced or been formed out of nephridial funnels.
The more complete replacements, such as the nephridia of the genital segment of Tubifex by a subsequently formed genital duct, may be compared with the succession of the nesonephros to the pronephros in vertebrates, and of the metanephros to the mesonephros in the higher vertebrates..
The genital ducts are limited to one segment (the 8th in Capitella capitata), and there are genital setae on this and the next FIG.
In other forms genital ducts and nephridia coexist in the same segment.
The Oligochaeta contrast with the Polychaeta in the general presence of outgrowths of the septa in the genital segments, which are either close to, or actually involve, the gonads, and into which may also open the funnels of the gonad ducts.
In dextral Gastropods the only structure found on the topographically right side of the rectum is the genital duct.
Here it lies close upon the genital body (ovary or testis), and in such intimate relationship with it that, when ripe, the gonad bursts into the renal sac, and its products are carried to the exterior by the papilla on the right side of the anus is FIG.
Represented by the genital duct.
The sexes are distinct, as in all Streptoneura; and genital ducts and accessory glands and pouches are present, as in all Pectinibranchia.
Y, Vesicle on genital duct.
Accessory organs are rarely found on the genital ducts, but occur in Paludina, Cyclostoma, Naticidae, Calyptraeidae, &c. Mandibles usually present.
Shell with short spire; lateral cervical lobes present; accessory genital glands.
In the most primitive condition the genital duct is single throughout its length and has a single external aperture; it is therefore said to be monaulic. The hermaphrodite aperture is on the right side near the opening of the pallial cavity, and a ciliated groove conducts the spermatozoa to the penis, which is situated more anteriorly.
The genital duct is now said to be diaulic, as in Valvata, Oncidiopsis, Actaeon, and Lobiger among the Bullomorpha, in the Pleurobranchidae, in the Nudibranchia, except the Doridomorpha and most of the Elysiomorpha, and in the Pulmonata.
The genital duct is now trifurcated or triaulic, a condition which is confined to certain Nudibranchs, viz.
F, The genital pore.
In front of the osphradium is the single genital pore, the aperture of the common or hermaphrodite duct.
Far from the genital pore a glandular c, Spermatheca.
Ing by means of a longish duct into a, Genital pore.
But when the male secretion of the ovo-testis is active, the seminal fluid passes from the genital pore along the spermatic groove (fig.
39) to the penis, and is by the aid of that eversible muscular organ introduced into the genital pore of a second Aplysia, whence it passes into the spermatheca, there to await the activity of the female element of the ovo-testis of this second Aplysia.
The common genital pore (male and female).
D, Genital pore.
Hermaphrodite genital aperture, connected with the penis by a ciliated groove, except in Actaeon, Lobiger and Cavolinia longirostris, in which the spermiduct is a closed tube.
Cephalic shield bifid posteriorly; margins of foot slightly developed; genital duct diaulic; visceral cornmissure streptoneur ous; shell thick, with prominent spire and elongated aperture; a horny operculum.
Shell external, globular or ovoid; foot elongated, parapodia separate from ventral surface; genital duct diaulic. Lobiger.
Genital duct monaulic; hermaphrodite duct connected with penis by a ciliated groove.
Genital duct diaulic, without open seminal groove; male and female apertures contiguous.
Genital duct diaulic; male and female apertures contiguous.
Genital duct triaulic. Spicules present in the mantle.
Y, Genital pore.