investment, and consist of an epithelium and an internal mass of branching cells continuous with the lacunar tissue in the visceral mass. [Illustration: FIG. 68.—Development of _Sacculina neglecta_. =A=, Nauplius stage, × about 70; =B=, Cypris stage, × about 70. _A_{1}_, _A_{2}_, 1st and 2nd antennae of Nauplius; _Ab_, abdomen; _Ant_, antenna of Cypris; _E_, undifferentiated cells; _F_, frontal horn; _G_, glands of Cypris; _H_, tendon of Cypris; _M_, mandible; _T_, tentacles. ] The developmental history of the Rhizocephala is one of the most remarkable that embryology has hitherto revealed. It has been most accurately followed in the case of _Sacculina_. The young are hatched out in great numbers from the maternal mantle-cavity as small Nauplii (Fig. 68, A) of a typical Cirripede nature, but without any alimentary canal. They swim near the surface of the sea, and become transformed into Cypris larvae of a typical character (Fig. 68, B). The Cypris larva, after a certain period of free existence, seeks out a crab and fixes itself by means of the hooks on its antennae to a hair on any part of the crab’s body. Various races of _Sacculina_ are known which infest about fifty different species of crabs in various seas; the best known are _S. carcini_ parasitic on _Carcinus maenas_ at Plymouth and Roscoff, and _S. neglecta_ on _Inachus mauritanicus_ at Naples. The antenna, by which the Cypris is fixed, penetrates the base of the hair; the appendages are thrown away, and a small mass of undifferentiated cells is passed down the antenna into the body-cavity of the crab. Arrived in the body-cavity it appears that this small mass of cells is carried about in the blood-stream until it reaches the spaces round the intestine in the thorax. Here it becomes applied to the intestine, usually at its upper part, immediately beneath the stomach of the crab (Fig. 69), and from this point it proceeds to throw out roots in all directions, and as it grows to extend its main bulk, called the central tumour (_c.t_), towards the lower part of the intestine. As the posterior border of the central tumour grows down towards the hind-gut, the future organs of the adult _Sacculina_ become differentiated in its substance; the mantle-cavity being excavated and surrounding the rudiment of the visceral mass, while as the central tumour grows downwards it leaves behind it an ever extending system of roots. When the central tumour in process of differentiation has reached the unpaired diverticulum of the crab’s intestine, at the junction between thorax and abdomen, all the adult organs are laid down in miniature, and the whole structure is surrounded by an additional sac formed by invagination known as the perivisceral space (Fig. 70). The young “_Sacculina_ interna” remains in this position for some time, and being applied to the ventral abdominal tissues of the crab just at the point where thorax and abdomen join, or a little below it, it causes the