For the observation of the means by which these animals attach themselves after leading a free life wo are also indebted to Mr. Dar win. In the larva, two ducts, called cement-ducts, can be traced from within the discs of the antenna, to the anterior or lower ends of the two gut-formed bodies, which are the incipient ovaries. These ducts are filled with an opaque cellular matter in the larva. In the mature. Cirripodee, they can be followed in a slightly sinuous course along the muscles on each side within the peduncle, till they expand into two email organs, which Mr. Darwin calls cement-glands. These glands contain a strongly coherent pulpy opaque cellular mass, like that in the cement-ducts; but in some instances this cellular mass becomes converted, within either the duets or gland, or within both, into a transparent tough yellow cement. Tubes are seen running into these, glands, containing ova in every stage of development From obser vations made on many species of Cirripedes, Mr. Darwin concludes that the gland itself is part of an ovarian tube specially modified; and further that the cellular matter, which in the ovarian tubes serves for the development of the ova, is, by the special action of the walls of the gland, changed into the opaque cellular matter in the ducts, and this again subsequently into that tissue or substance which cements, the Cirripede to its surface of attachment. An the individuals grow and increase in size, so do the glands and cement-ducts; but it often happens that when a specimen is immoveably attached, the cement apparatus ceases to act., and the cellular contents of the duct become converted into a thread of transparent cement The cement removed from the outside of a Cirripede consists of a thin layer of very tough bright-brown transparent laminated substance, exhibiting no structure under the microscope. Its chemical reactions are those of Chitino. In the larva, the cement always escapes through the prehensile antennae, and in most instances it continues to do this throughout the life of the animal. There are however exceptions, and in Sealpellumndgere, and probably others which live attached to coral, the cement soon ceases to dobouch from the antenna,, but instead bursts through a row of orifices on the mistral margin of the peduncle, by which means this margin is symmetrically fastened down to the delicate horny branches of the zoophyte.
The external shell, which misled early observers, and induced them to place the Cirripedes among the Molluaca is in the Lepadidm, the Capitulum. It is usually much flattened, but sometimes broadly oval in section. It is generally formed of five or more valves, connected together by very narrow or broad strips of membrane. When the valves are numerous, and they sometimes exceed a hundred in number, they are arranged in whorls, with each valve generally Replaced as to cover the interval between the two valves above. The upper pair of valves, the peduncle being beneath, are called by Mr. Darwin the terga; the pair below it, on the same side, the acute. The upper mesinl valve opposed to the two terga is the caring, and below this the aubcarine; and on the opposite side again the rostrum. Below this is sometimes a aubrostrum. Of all the valves the acute are the most permanent ; then come the terga, and then the mina. The others occur only occasionally. The shell is
generally white, occasionally reddish or purple ; exteriorly the valves are covered by more or less persistent, generally yellow, strong mem brane. The acute and terga are always considerably larger than the other valves. The adductor muscle is always attached to a point not far from the middle of each scutum, which generally has a pit for its attachment. The valves are either placed close together or at some dis tance. The membrane connecting the valves, where they do not touch, is like that forming the peduncle, and is sometimes coloured brilliantly crimson-red ; generally it is bluish-gray. Within the capitulum is the sac, which, together with tho upper internal part of the peduncle, inclosed the animal's body.
The Peduncle varies in length in different species, and even in the same species, according to the situation occupied by the individual. It in usually flattened, but sometimes quite cylindrical. It is com posed of very strong generally thick transparent membrane, rarely coloured reddish, and often penetrated by numerous tubuli. The peduncle is lined within by thin layers of muscles, longitudinal, transverse, and oblique, all destitute of the transverse stria, charac teristic of voluntary muscles. They run from the bottom of the peduncle to the base of the capitulum, as in Lepaa, or half way up it as in Conchmkrma. The gentle swaying to and fro movements and the power of longitudinal contraction are produced by these muscles. The interior of the peduncle is filled up with a great mass of branching ovarian tubes.
There arc six pairs of cirri. The five posterior pairs are seated close to each other and equidistant; the first pair is generally seated nt n little distance and sometimes at a considerable distance from the second pair. The first pair is shortest; the others, proceeding back wards, increase gradually in length. The number of segments in the posterior cirri is very great The cirri are covered with spines. Most of the genera have caudal appendages.
The alimentary canal consists of an cesophagun, a stomach, and rectum. The oesophagus is of considerable length ; it is formed of strong transparent much-folded membrane, contintioun with the outer integuments, and moulted with them. At its lower end it expands into a bell with the edges reflexed. Thin bell lies within the stomach, and keeps the upper broad end expanded. The stomach lies in a much-curved almost doubled course, and is often a little constricted where most bent. It ie broadest nt the upper end. The stomach is coated by small opaque pulpy slightly arboreacent glands, believed to be hepatic. The rectum varies in length, extending inwards from the antis to between the bases of the second and fifth pair of cirri. It is narrow, and formed of much-folded transparent membrane. Within the stomach there can generally be seen, accord ing to the period of digestion, a thin yet strong perfectly transparent einthelinl membrane, not exhibiting under the microscope any !true ture. It enters the branching mem, and extemls from the edge of the hell of the (esophagus to the commencement of the closed rectum. It was this membrane which was supposed by M. Martin-Saint-A nge to be a distinct organ, like the closed tube of certain A nuclide..