ANNULOSA: CRUSTACEA. 289 



mouth, and there is a well-developed alimentary tube, which terminates by 

 a distinct anus at the root of the caudal spine. There is at first merely a 

 simple central eye ; but in the adult Nauplius, two compound lateral eyes 

 are developed in addition. Ultimately, the Nauplius passes into its second 

 condition or " Cypris-stage " (fig. 143, B), when it is often spoken of as a 

 "pupa." It is now enclosed in an oval, bivalved, mussel-shaped shell, 

 with an opening along the ventral margin. The second and third pairs of 

 the appendages of the Nauplius have now disappeared, and the first pair 

 of appendages constitute strong four-jointed antennae, the last segment of 

 which is disc-shaped, and is pierced centrally by a pore, which is the open- 

 ing of the excretory duct of the ' ' cement-glands, " these organs being situated 

 at the bases of the antennae. The thorax has developed upon its sides six 

 pairs of forked natatory limbs ; and the abdomen is rudimentary, three- 

 jointed, with terminal forked swimming-appendages. The pupa does not 

 feed, but is nourished by means of an extensive accumulation of fatty matter, 

 which had been stored up by the Nauplius in the cephalic and dorsal regions 

 of the body ; while the great labrum of the latter is now very much reduced 

 in size. 



After a brief natatory life, the pupa fixes itself by means of the disc-seg- 

 ments of the antennae to some foreign body, such as a rock, a piece of drift- 

 wood, the skin of a Cetacean, a Sponge, the carapace of a Turtle, or the 

 colony of an Oceanic Hydrozoon. The "cement -glands," which, as 

 shown by Darwin, are "part of and continuous with the branching ovaria," 

 secrete copiously an adhesive cement, which is poured forth through the 

 central apertures of the antennal discs, and by means of which the animal 

 is firmly and finally fastened down to the object to which it in the first 

 place attached itself. The body now becomes enclosed in a multivalve 

 calcareous "test," produced by a special shell-gland. The organs of the 

 mouth become fully developed, and the lateral eyes of the locomotive pupa 

 disappear altogether. Lastly, the six pairs of natatory limbs of the Cypris- 

 stage are replaced by the six forked and multisegmentate "cirri" of the 

 adult ; while the base of the abdomen carries the penis, in the form of a 

 proboscidiform appendage. 



The form of the adult, as already said, differs considerably, 

 but the two most important types are those presented respec- 

 tively by the Sessile and by the Pedunculated Cirripedia. 



In the symmetrical Sessile Cirripedes or Balanidce, commonly 

 known as Acorn-shells (fig. 142, C, D), the animal is protected 

 by a calcareous shell, formed by calcifications within the walls 

 of the first three cephalic segments. The animal is placed 

 within the shell, head downwards, and is fixed to the centre of 

 a shelly or membranous plate, which closes the lower aperture 

 of the shell, and which is termed the "basis." The "basis " is 

 fixed by its outer surface to some foreign object, and is some- 

 times compact, sometimes porous. Above the basis rises a 

 limpet-shaped, conical, or cylindrical shell, which is open at 

 the top, but is capable of being completely closed by a pyra- 

 midal lid or " operculum." Both the shell itself and the oper- 

 culum are composed of calcareous plates usually differing from 

 one another in shape, and distinguished by special names. 

 Within the shell the animal is fixed, head downwards. The 



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