The development of the Cirripedia is of special interest, since it was the discovery of the larval stages by J. Vaughan Thompson that first demonstrated to naturalists that the Barnacles were Crustacea and not, as had been supposed, Molluscs. The earliest stage is generally a nauplius (Fig. 35, A) of very peculiar and characteristic form, with a pair of horns projecting sideways from the front corners of the dorsal shield, and a forked spine on the under-side behind. The later development is very unlike those which have been described above, for after a series of nauplius stages the larva passes suddenly, at a single moult, into a stage in which the body and limbs are enclosed in a bivalved shell (Fig. 35, B). From the superficial resemblance of the shell to that of an Ostracod, this is known as the _cypris_ stage. Through the valves of the shell a pair of large compound eyes can be seen, as well as six pairs of two-branched swimming feet, while in front a pair of antennules project between the valves. On each antennule is a sucker-like disc by means of which the larva, after swimming freely for some time, attaches itself to a stone or some other object, where it remains fixed for the rest of its life. A cementing substance produced by a gland at the base of the antennules attaches the front part of the head firmly to the support; the valves of the shell are cast off, and replaced by the rudimentary valves of the adult shell; the six pairs of swimming feet grow out into tendril-like cirri; the compound eyes disappear, and the animal assumes the structure of the adult. The parasitic Rhizocephala have a very remarkable life-history, which will be described in a later chapter; but it may be mentioned here that their free-swimming larval stages resemble very closely those of the ordinary Barnacles. It was the discovery of this fact which led to its being recognized that the Rhizocephala are highly modified and degenerate Cirripedes, although their structure in the adult state gives little evidence of their affinities. A number of interesting problems in speculative biology are suggested by the larval stages of Crustacea. A full discussion of these problems would involve matters too technical for these pages, but some indication of the broader issues may be attempted.