AI4 THE ORIGIN OF VERTEBRATES 
the body a segmented pleuron, is exactly in accordance with the 
theory of the origin of vertebrates deduced from the study of 
Ammoccetes, as already set forth in previous chapters. For we see 
that one of the striking characteristics of such forms as Bunodes, 
Hemiaspis, etc., is the presence of segmented pleural flaps on each 
side of the main part of the body; and if we pass further back to the 
great group of trilobites, we find in the most manifold form, and 
in various degrees of extent, the most markedly segmented pleural 
folds. In fact, the hypothetical figure (Fig. 160, A) which I have 
deduced from the embryological evidence, might very well represent 
a cross-section of a trilobite, provided only that each appendage of 
the trilobite possessed an excretory coxal gland. 
The earliest fishes, then, ought to have possessed segmented 
pleural folds, which were moved by somatic muscles, and enveloped 
the body after the fashion of Ammoccetes and Amphioxus, and I 
cannot help thinking that Cephalaspis shows, in this respect also, its 
relation to Ammoccetes. It is well known that some of the fossil 
representatives of the Cephalaspids show exceedingly clearly that 
these animals possessed avery well-segmented body, and it is equally 
recognized that this skeleton is a calcareous, not a bony skeleton, 
and does not represent vertebra, etc. It is generally called an 
aponeurotic skeleton, meaning thereby that what is preserved repre- 
sents not dermal plates alone, or a vertebrate skeleton, but the calcified 
septa or aponeuroses between a number of muscle-segments or 
myomeres, precisely of the same kind as the septa between the 
myomeres in Ammoccetes. The termination of such septa on the 
surface would give rise to the appearance of dermal plates or scutes, 
or the septa may even have been attached to something of the nature 
of dermal plates. The same kind of picture would be represented if 
these connective tissue dissepiments of Ammoccetes were calcified, 
and the animal then fossilized. In agreement with this interpre- 
tation of the spinal skeleton of Cephalaspis, it may be noted that 
again and again, in parts of these dissepiments, I have found in old 
specimens of Ammoccetes nodules of cartilage formed, and at trans- 
formation it is in this very tissue that the spinal cartilages are 
formed. 
Now, the specimens of Cephalaspis all show, as seen in Fig. 161, 
that the skeletal septa cover the body regularly, and then along one 
line are bent away from the body to form, as it were, a fringe, or 
