Thus the sessile Cirrhipedes, with their mollusc-like shells, their soft, unsegmented bodies, degenerate heads, and their twelve vibratile food-wafting limbs, emerge from the egg as nauplius larvæ. But the remarkable parasites on the shore-crabs and the hermit-crab deviate much further from the type of the rest of the Crustaceans, for they hang like a sac or formless sausage-like soft mass to the abdomen of their host, growing into it by fine, pale, root-like threads, through which they suck up the blood of their hosts (Fig. 112, _C. Sacc._). They possess neither head, nor thorax, nor abdomen, not even an indication of segmentation, no limbs of any kind, neither antennæ, nor mouth parts, nor swimming-legs. Nevertheless they are Crustaceans; indeed, we can say with certainty that they belong to the order of Cirrhipedes, for they leave the egg in the form of a nauplius larva (_A_), with 'horns' on their carapace which no other forms except themselves and the Cirrhipedes possess. That they are of the same stock as these is also proved by their further development, for the nauplius grows first, just as in the case of the Cirrhipedes proper, into a 'Cypris-like larva' (_B_), so called because it bears a certain resemblance to the Ostracods of the genus Cypris, and only from this point do their paths of development diverge. The Cypris-like larva of the true Cirrhipedes settles down somewhere, attached by its antennæ; it grows, and its body becomes that of the perfect Cirrhipede; but the Cypris-like larva of the Sacculinæ bores its way into the inside of a crab or hermit-crab, at the same time losing its limbs, segmentation, and its chitinous covering; and within the body of its host it is transformed into the sac-like organism we have already described. After a time it emerges again on the surface, and remains attached to the abdomen of its host (Fig. 112, _C. Sacc._), drawing its nourishment from the blood which it sucks up by means of its numerous delicate roots (_W_, _W_). [Illustration: FIG. 112. Development of the parasitic Crustacean _Sacculina carcini_, after Delage. _A_, Nauplius stage. _Au_, eye. _I_, _II_, _III_, the three pairs of appendages. _B_, Cypris-stage. _VI_-_XI_, the swimming appendages. _C_, mature animal (_Sacc_), attached to its host, the shore-crab (_Carcinus mænus_), with a feltwork of fine root-processes enveloping the crab's viscera. _s_, stalk. _Sacc_, body of the parasite. _oe_, aperture of the brood-cavity. _Abd_, abdomen of the crab with the anus (_a_).] From all this we may conclude that certain Cirrhipedes in times long past adopted a parasitic habit in the Cypris-larva stage, and that they gradually underwent adaptations to this mode of life, and that these went further and further, until the animal was transformed into the singular creature which we now see in the sexually mature form.