17 1 Comparative Morphology of Chordates 



nephros, which persists as one of the pair of functional kidneys of the 

 adult. In males the anterior part of the mesonephros becomes con- 

 nected with the testis of the same side, and sperm passes through the 

 mesonephric tubules into the outgoing duct (Wolffian duct) of the 

 kidney (Fig. 78). The posterior part of the mesonephros serves only 

 the renal function. The duct, therefore, is not merely urinary, but is a 

 urinogenital duct. In females the mesonephros has nothing to do 

 with the ovary or eggs. 



The reptilian embryo develops structures which agree in all essen- 

 tials with the pronephros and mesonephros, but neither of them per- 

 sists to the adult stage except that in males the anterior part of the 

 mesonephros becomes transformed into the "accessory testis" or 

 epididymis (Fig. 78C) which intervenes between the testis and the 

 sperm-duct (vas deferens). The definitive kidneys of the adult reptile 

 are a third pair, each of which (a metanephros) develops in close 

 relation with, but posterior to, the embryonic mesonephros. The 

 reptilian kidney, therefore, is merely analogous to the anamniote 

 kidney, but the two are not homologous. (It is possible, but doubtful, 

 that the hind part of the kidneys of some elasmobranchs and amphib- 

 ians may be of metanephric nature. See p. 85.) 



In all reptiles the urinogenital ducts and the anus have exit by way 

 of a common superficial chamber, the cloaca. A urinary bladder, 

 opening out from the midventral wall of the cloaca, may or may not 

 be present. In the New Zealand tuatara, a supposedly "primitive" rep- 

 tile, the bladder is especially large. It occurs in turtles and tortoises 

 and in most lizards. It is lacking, among lizards, in the monitors and 

 legless lizards (amphisbaenians), and is absent also in snakes, alli- 

 gators, and crocodiles. 



The nervous organs of reptiles are, in general, similar to those of 

 amphibians, but there are numerous differences which, although not 

 conspicuous anatomically, do indicate a higher degree of nervous 

 efficiency in the reptile. The most striking difference consists in the 

 absence of lateral-line organs. No trace of them is found in the 

 adult reptile or in the embryo. Constructed as they are in anamniotes, 

 they could not endure exposure to dry air and impact of hard objects, 

 and their presence in reptilian skin would be quite incompatible with 

 the development of a horny layer. They are peculiarly adapted to an 

 aquatic environment. Any disadvantage which the reptile might suffer 

 in their absence is doubtless fully compensated for by improvements 

 in other sensory organs. Equally incompatible with the horny layer 

 would be organs such as the integumentary organs of "taste" as they 

 occur on the bodies of fishes. 



