414 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 Ammocoetes 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 a very well-segmented body, and it is equally 

 recognized that this skeleton is a calcareous, not a bony skeleton, 

 and does not represent vertebrse, 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 Ammocoetes 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 AmmocGetes 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 



