The longitudinal muscles, the nerves, and excretory vessels which supply the proglottides are continuous throughout and with those of the head. Each joint contains at first male genitalia comparable with those of a Trematode; then the female organs develop, and finally self-fertilisation follows. The Cestodes feed through their skin, probably by the aid of fine protoplasmic processes, which penetrate the tough investing membrane and absorb the already digested food which bathes them. When a proglottis of _Calliobothrium_ is approaching maturity it separates from the parent, the broken ends of muscles, nerves, and excretory vessels speedily heal, and it is now capable of continued growth and of fairly active movement if it remains in the intestine of the host. According to van Beneden, it may even attain a size equal to, or exceeding, that of the whole parent or "strobila."[97] These considerations led Leuckart, von Siebold, P. J. van Beneden, and others, to Steenstrup's conclusion that a jointed tape-worm is really a colony composed of two generations—the head and neck derived from the larva, and the proglottides produced by the segmentation of the neck.[98] This view of the colonial nature of jointed Cestodes was generally adopted from 1851 to 1880. During the last fifteen years, however, the varied interpretations of the facts of the ontogeny of this group have led some authors to adopt the monozootic view (that a Cestode is one individual), others are still of the older opinion, and Hatschek (_Lehrbuch_, p. 349) and Lang take up intermediate positions. Lang considers that the formation of the joints of a tape-worm from a small fixed "scolex," is not only largely comparable with the strobilation of a _scyphistoma_ and the consequent formation of a pile of medusae, as in the life-history of _Aurelia_, but {77}that both processes have arisen from the power of regenerating the necessary organs in each of the new segments. The result in both cases is the rapid formation of a number of joints, which gradually separate from the parent, to carry the eggs and young to new stations. Just as some Coelenterata (_Lucernaria_) may be regarded as not having advanced much beyond a scyphistoma stage, so there are unisegmental Cestodes (e.g. _Archigetes_, Fig. 37) which have remained as a slightly altered but sexual scolex, directly comparable with a Trematode, and, as all authors are agreed, representing one generation only. Such monozootic forms are now classed as a special family, the Cestodaria or Monozoa, of which _Caryophylleus mutabilis_, from the intestine of various Cyprinoid fish, is the most abundant representative, while _Amphiptyches_ (_Gyrocotyle_) _urna_, from _Chimaera monstrosa_ of the northern hemisphere, is paralleled by _A. rugosa_, found in _Callorhynchus antarcticus_ of the southern seas.