THE GILL SACS. 263 



the mouth, but finally coming to lie inside of it, apparently on the premaxillary 

 lobes (Figs. 171, 172.) Premaxillary discs also occur in Lepidopterus and Amia. 



There can be but one inference from the facts that have been enumerated, 

 namely, that the so-called " visceral arches" of vertebrates represent the basal 

 lobes of the cephalo-thoracic appendages of their arthropod ancestors and; that the 

 external gills, the balancing organs of amphibia, the cephalic oars of ostracoderms, 

 the tentacles, and adhesive discs of the oral arches in embryonic fishes and 

 amphibia are serially homologous structures, representing the remnants of the 

 cephalothoracic appendages themselves. 



Similar modifications of the appendages frequently occur in various classes 

 of arthropods. For example, in insect embryos (Blatta and Acilius) the first pair 

 of abdominal appendages are reduced to gland-like discs or cups, very similar 

 in appearance to the cement glands on the oral arches of embryo fishes and 

 amphibia. Beyond this, their function and significance is unknown. In many 

 Crustacea, entomostraca, and cirripeds, one or two pairs of degenerate cephalic 

 appendages terminate in adhesive discs by means of which they fasten themselves 

 to foreign objects, or to their hosts, just as larval vertebrates use their cement 

 glands for a similar purpose. l 



It remains to be seen how far a more detailed study of the structure and 

 growth of the oral arches in primitive vertebrates will confirm the above interpre- 

 tation. So far as we know, there is nothing in the embryonic history of the meso- 

 dermic head cavities, or of the oral arch nerves and ganglia, that conflicts with it, 

 while all the data available concerning the superficial form and the mode of 

 growth of these organs lend it their unqualified support. 



V. THE GILL SACS. THE THYROID AND THE THYMUS. 



In many fishes and amphibia the external gills disappear and their place is 

 taken by internal gills developed at a later period from the walls of gill sacs or 

 pouches. The latter are formed between the gill arches by an infolding of the 

 ectoderm that unites with a tubular outgrowth from the mesenteron. It is not 

 clear whether the new gill lamellae arise solely from the entodermic, or from the 

 ectodermic part of the gill chamber, or from both, although the prevalent opinion 

 strongly favors the first alternative. 



However that may be, it is a fact, the significance of which is readily apparent, 

 that in the arachnids there is the same kind of gill sacs, having the same relation 

 to external appendages, located in the same region of the head, and having the 

 same relation to enteric outgrowths, as in the embryos of primitive vertebrates. 



1 The origin of the secreting surface of the cement glands from the endoderm is not in conflict with this 

 view. It merely confirms the interpretation of the several oral arches as serially homologous with the gill 

 arches. 



