Note 27 — Chapter V · THE MORPHOLOGICAL COMPOSITION OF ANIMALS, CONTINUED.

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.