[23] I find that the reasons for regarding the segment of a Tænia as answering to an individual of the second order of aggregation, are much stronger than I supposed when writing the above. Van Beneden says:—“Le Proglottis (segment) ayant acquis tout son développement, se détache ordinairement de la colonie et continue encore à croître dans l’intestin du même animal; il change même souvent de forme et semble doué d’une nouvelle vie; ses angles s’effacent, tout le corps s’arrondit, et il nage comme une Planaire au milieu des muscosités intestinales.”
[24] Though this was doubtful in 1865 it is no longer doubtful. In an individual Ctenodrilus monostylus, which multiplies by dividing and subdividing itself, “parts arise which are destitute of both head and anus and at times consist of only a single segment.” In another species, C. pardalis, there is separation into many segments; and each segment before separating forms a budding zone out of which other segments are afterwards produced, completing the animal (Korschelt and Heider, Embryology, i, 301–2).
[25] In place of those originally here instanced about which there are disputes, I may give an undoubted one described by McIntosh, the Syllis ramosa, a species of chætopod living in hexactinellid sponges from the Arafura Sea, which branches laterally repeatedly so as to extend in all directions through the canals of the sponge. In most cases the buds terminate in oval segments with two long cirri each. But male and female buds were found, provided each with a head, and containing ovaries and testes. Sometimes these sexual buds had become separate from the branched stock.
[26] The name Annulosa, once used to embrace the Annelida and Arthropoda, has of late ceased to be used. It seems to me better than Appendiculata, both as being more obviously descriptive and as being more exclusive.
[27] The fusion of the segments forming the Arthropod head and the extreme changes, or perhaps in some cases disappearances, of their appendages, put great difficulties in the way of identification; so that there are differences of opinion respecting the number of included segments. Prof. MacBride writes:—“It is highly probable that a primary head (præoral lobe or præstomium) has been derived from annelid ancestors, but the secondary fusion of body-segments with this head, in other words the formation of a secondary head, has gone on independently in the different classes of the phylum Arthropoda, viz., Arachnida, Crustacea, and Tracheata (including Insects and Myriapods). Judged by the number of appendages (which gives an inferior limit) the head of a malacostracous Crustacean consists of præstomium and 8 segments; the head of an insect of præstomium and 4 segments; the head of a Myriapod of præstomium and 3 segments; and the head of an Arachnid of præstomium and 3 segments.” Again, the comment of Mr. J. T. Cunningham is:—“According to Claus and most modern authorities there are only 5 segments in the head of an Arthropod, the eyes not counting as appendages; and further it should be noted that the second pair of antennæ are wanting in Insects.”
Of course difference of opinion respecting the number of somites in the head involves difference of opinion respecting the number constituting the entire body, which, in the higher Arthropods, is said by some to be 19 and by others 20. But those who thus differ in detail, agree in regarding all the segments of head and body as homologous, and this is the essential point with which we are here concerned.
[28] Prof. MacBride corrects this statement by saying that “The ctenidia or gills (which in Mollusca generally are represented only by a single pair) are here represented by a large number of pairs; they do not, however, correspond in either number or position to the shell plates.” It may, I think, be contended that if these had any morphological significance, they would not differ in arrangement from the shell plates, and would not be limited to this special type of Mollusc.
[29] Though it is alleged that at a later stage the posterior part of the skull is formed by fusion of divisions which are assumed to represent vertebræ, yet it is admitted that the anterior part of the skull never shows any signs of such division. Moreover in both parts the bones show no trace of primitive segmentation.
[30] See note at the end of the chapter.
[31] A qualifying fact should be named. When the production of vertebral segments has become constitutionally established, so that there is an innate tendency to form them, there arises a liability to form supernumerary ones; and this, from time to time recurring, may lengthen the series, as in the body of a snake or the neck of a swan. This qualification, however, affects equally the hypothesis of an ideal type and the hypothesis of mechanical genesis.