The spiracle serves as a point of entry for respiratory water 

 in the case of the rays. The pharyngocutaneous duct might 

 serve this purpose in the case of the myxinoid. In both kinds 

 of cyclostomes the muscular pouches are capable of produc- 

 ing a pumping action with the result that water could enter 

 the pharynx either from the mouth or the hypophyseal duct 

 — the pharyngocutaneous duct in the case of the myxinoid. 

 Water may be drawn into the pouch through its outer 

 opening in the lamprey, then pumped from the pouch 

 through the same port. 



The branchial systems of the cyclostomes differ in their 

 vascular relationships but agree in the general form of the 

 pouches (restricted internal and external ports). The median 

 septum of the arch in this group is a complex wall between 

 pouches. It is possible that the marsipobranch type of pouch 

 preceded the narrow branchial bars of the gnathostome, but 

 it is just as likely that it represents a respiratory specialization 

 related to the agnathan mouth. As a specialization it cannot 

 be considered primitive or ancestral to the gnathostome. 



Fossil agnaths The number of pharyngeal pouches and 

 external branchial openings varies in the several fossil types. 

 In the osteostracan there were 1 or 11 pouches and openings, 

 the anaspids had from 8 to 15 openings, and the pteraspids 10 

 to 14 chambers all opening through a single external port. 

 The pouches in these fossils, in terms of preserved skeletal 

 clews, appear to have been like those of the living cyclo- 

 stomes. 



Genera/ observafions A branchial system was probably 

 present in the ancestral vertebrate, since pharyngeal slits are 

 a basic chordate feature. The primitive system can be im- 

 agined as a series of slits of indeterminate number, probably 

 not greater than seven, supported by some sort of connec- 

 tive tissue skeleton. It is possible that each branchial arch 

 was a holobranch, with the exceptions of the first and last. 

 The pouches of the agnath contrast sharply with the slits of 

 the gnathostome and suggest that they are modifications, 

 part of a pumping mechanism associated with thejawless 

 mouth. The gnathostome retained the simple slits but pro- 

 duced an articulated branchial skeleton, a part of which was 

 involved in the jaws. From this line of thought it seems 

 plausible that both agnath and gnathostome types are 

 divergences from the ancestral style. 



The many variations in number of pouches or slits and in 

 the details of structure indicate the plasticity of this region. 

 The Osteichthyes are characterized as having five gill slits, 

 as do most sharks and perhaps most of the arthrodires and 

 acanthodians, but these are covered by an operculum. The 

 actinopterygian differs from the choanate in having a pseudo- 

 branch. Polypterus appears to be quite distinct in most features. 



One might ask here if there were two evolutionary lines of 



fishes, one without an operculum, the primitive state, and one 

 with. Since the operculum is a part of a respiratory mecha- 

 nism, which directs water in through the mouth and out 

 past the gills, it is quite possible that this structure could 

 arise in several lines. The opercula of the osteichthian, 

 acanthodian, and holocephalan are distinctive in their 

 structural details and, probably, in their action, and thus 

 could be parallelisms. However, if the opercular skeletons 

 and action of these opercula were developed late, then all 

 could have come from a single ancestral source (monophy- 

 letic). The view that they are parallelisms (polyphyletic) 

 seems more in line with the facts. 



Body surface 



The general body surface is frequently used by vertebrates, 

 either as larvae or as adults, for respiration and in some has 

 become the main respiratory system, along with the lining 

 of the mouth. It is generally assumed that the most primitive 

 vertebrates, based upon the porous nature of their dermal 

 bones, had a rather rich capillary net associated with the 

 outer part of the dermis and that this capillary net supplied 

 a certain amount of the respiratory requirements for these 

 organisms. 



The amphibian stage apparently marked the dividing 

 point for respiratory pathways. The living amphibians re- 

 present forms in which cutaneous respiration was continued 

 or even further developed, as in the case of the lungless 

 salamanders, while the reptiles represent a type in which 

 the outer layer of the skin became cornified or keratinized 

 and thus impermeable or only slightly permeable to the re- 

 spiratory gases. This keratinization was important in com- 

 bating drying. Whereas the moist-skinned amphibian would 

 be limited to wet areas or places of high humidity, the dry- 

 skinned reptile could seek food anywhere within the ranges 

 of its temperature tolerances. This dichotomy of the verte- 

 brate stem was extremely important in the evolution of this 

 group. 



RESUME 



The viscera of the vertebrates suggest some of the basic 

 events of their phylogeny, However, this record is difficult 

 to assess because of the functional plasticity of these organs 

 and systems. Again the basic split into agnath and gnatho- 

 stome is seen and the several groups of gnathostomes are 

 suggested. The dissimilarity of shark and holocephalan is 

 emphasized again as is the unique nature of Polypterus. Re- 

 semblance between the dipnoan and amphibian is slight be- 

 cause of the modified nature of the former. Comparisons 

 would probably be better with a Devonian dipnoan. The 

 tetrapod groups agree closely in most details. 



290 



THE VISCERA 



