The adult Sacculina is known as Sacculina externa which is a parasite of young decapod crustacean, the crab. Generally the female crab is parasitized by the Sacculina. The parasitic habit has caused much degeneracy of different structures in the adult and looks like a fleshy tumour.
The adult looses all the external appendages, the carapace and the segmentation of the body. The mouth and anus are absent. It is seen as a soft round tumour or ovoid mass on the ventral side of the abdomen of a female crab. It remains attached to the crab by a peduncle.
From this tumour numerous branched filaments are ramified within the body of the host, except the heart and the gills. The sac like body or round tumour opens to the exterior by a genital or cloacal aperture. This cloacal aperture opens within the body into a brood cavity. The wall which surrounds the brood cavity is called mantle. Within the brood cavity is a central visceral mass which is finger like projection.
In adult Sacculina all the organ systems are absent except the nervous and reproductive systems. The peduncle sends numerous root-like filaments into the body of the crab which may extend even into the legs and antennules.
Through them, the parasite draws the nourishment and expels the waste as well as the excretory products. The anterior visceral mass comprises of a nerve ganglion. The nerve ganglion supplies nerves to the elongated visceral mass and to the mantle wall through the mesentery.
The parasite is hermaphrodite or monoecious i.e. the male and female reproductive organs are found in the same animal. The visceral mass bears a pair of testes and a pair of ovaries. The ovaries are asymmetrical, lie at the middle of the visceral mass and open by oviducts directly. The ovaries expel the eggs into the mantle cavity.
The testes are saccular and numerous. Their ducts open into the mantle cavity. The visceral mass also bears a pair of accessory genital glands i.e. cementing glands. A pair of atrium and a pair of collateral glands are housed inside the anterior visceral mass (Fig. 14.1).
