The alimentary canal is simple, but can be distinctly divided into--(1st) an œsophagus, singular from the bell-shaped expansion of its lower end; (2d) the stomach, which is directed forwards and then doubled back; and (3d) the rectum. There is no distinct liver. The circulation is lacunal. In one family there are well-developed branchiæ, which differ entirely in their homologies and position from these organs in all other Crustaceans. In the nervous system, the sub-œsophageal ganglions vary in concentration from that degree observed in the lower Macroura, to that in the highest Brachyoura; but the supra-œsophageal ganglions are always much less concentrated, and are even embryonic in condition; presenting a difference not observed in other Crustaceans. On the under side of the sub-œsophageal ganglion, two nerves, apparently splanchnic, arise, and run almost parallel and under the collar surrounding the œsophagus; they are very remarkable from their great size, and from forming a plexus together with a large branch, arising on each side from the collar close behind the supra-œsophageal ganglion,--a structure unlike anything observed in other Crustaceans. The eyes, as already remarked, are rudimentary, and singular from being imbedded at a distance from the anterior end of the animal. In the basal confluent segments of the outer maxillæ there are two orifices, leading into little sacks, which I believe are olfactory organs: again there are two other orifices on each side of the thorax, beneath the first pair of cirri, leading into sacks, with a curious elastic vesicle suspended within them; and these I can hardly doubt are acoustic organs. Of these orifices and organs, there is no trace in the same relative positions in any known Crustacean. Cirripedes are ordinarily bisexual, in which they differ from all Crustaceans: when the sexes are separate, the males are minute, rudimentary in structure, and permanently epizoic on the females; to these latter facts we have a partial analogy in some of the Suctorial Entomostracans; but a far closer analogy in certain Rotifers, which are considered by many naturalists as Crustaceans; but to the above subject I shall almost immediately have to recur.