174 METABOLIC HORMONES 



from a larval ammocoete, which has a pharyngeal endostyle 

 resembling that of the Protochordata, to an adult form with a 

 thyroid comparable with that of other vertebrates. The endostyle 

 of ammocoetes is capable of accumulating and storing iodine, 

 associated with a glycoprotein, but the organ does not secrete the 

 colloid, typical of a thyroid gland, until the adult stage. Leach 

 (1946) found no evidence of changes in oxygen consumption 

 accompanying these secretory changes ; but he did not remove the 

 gland nor try the action of extracts. 



Elasmobranchii. Dogfish and skates have well-defined thyroid 

 glands, which can be removed relatively easily. Extracts of thyroid 

 from the dogfish, Scyliorhinus {= Scyllium), increase the oxygen 

 consumption of mammals (Fig. 5-3) ; but the presence of thyroxine 

 in the blood of these fish seems to have no discernible effect upon 

 their own metabolism. For a period of 42 days after thyroidectomy, 

 the fish showed no appreciable changes in their oxygen consump- 

 tion, as compared with mock-operated controls (Matty, 1954). 



Teleostei. Most teleost fish have diffuse thyroid tissue which is 

 recognizable histologically, but almost impossible to remove 

 surgically, except from the parrot-fish, Pseudoscariis, in which the 

 gland is in a compact capsule. Like the thyroxine of elasmobranchs, 

 that of teleosts is active in mammals (Fig. 5-4; D. C. Smith and 

 Brown, 1952). 



Earlier reports claim that the bony fish are as insensitive to the 

 metabolic effects of thyroxine as the foregoing groups, at least 

 when the thyroxine is of mammalian origin (Root and Etkin, 1937). 

 More recently a slight increase in oxygen consumption has been 

 induced in Bathystoma, by injecting extracts of thyroid from 

 another teleost fish, Pseudoscariis (Smith and Matthews, 1948). 

 It has also been found that synthetic thyroxine and thyroid 

 stimulation by a thyrotropic preparation (§4.221) both increase 

 the oxygen consumption of goldfish, Cyprinus, by as much as 100 

 per cent for as long as 5 hr (Miiller, 1953). Despite the well-known 

 sensitivity of fish to handling, which makes experimentation 

 extremely difficult (Hoar, 1957), control fish showed only 20 per 

 cent increase in oxygen consumption, lasting only 40 min, as a 

 result of saline injections. 



Amphibia. Seasonal changes in size and activity of the thyroid 



