§ 5.111 RESPIRATION 173 



It is possible that the effect measured here should be associated 

 with the diabetogenic action of the corpora allata (§ 5.211). 



The action of the corpus allatum that increases oxygen consump- 

 tion is not stimulated by nerves ; nor is there any definite evidence 

 of an endocrinokinetic effect (§ 4.213) upon them from either the 

 median or lateral cerebral neurosecretory cells or their storage 

 organ, the corpus cardiacum (E. Thomsen, 1952). 



Chordata. Thyroxine secreted by the thyroid gland (§ 2.221) 

 is the hormone that is effective in many chordates in raising the 

 basal metabolic rate and increasing oxygen consumption ; but it is 

 best known in birds and mammals. The hormone contains iodine, 

 and is not highly specific, since extracts from the thyroid glands of 

 dogfish, Scyliorhinus, have almost the same effect upon mammals 

 as do extracts of their own glands (Fig. 5-3). Although thyroxine 

 is obtainable from many of the cold-blooded vertebrates, it is not 

 always clear that it has any physiological function in them, or has 

 at best more than seasonal significance in regard to metabolic rate. 

 In many cases thyroxine acts mainly as a morphogenetic hormone 

 (Part II, § 3). 



Protochordata. The Urochordata and Cephalochordata have 

 a ciliated endostyle in the floor of the pharynx, the primary 

 function of which is the secretion of mucus ; but on embryological 

 grounds the structure was, at one time, believed to be homologous 

 with the thyroid of vertebrates. 



Urochordata. The ascidian, Perophora, has been examined to 

 see if any of the tissues would accumulate radioactive iodine, as 

 the thyroid gland does. This examination gave the anomalous 

 result that the endostyle did not accumulate iodine, although the 

 stolon did so (Gorbman, 1941). 



Cephalochordata. Unlike the ascidians, the amphioxus, 

 Branchiostoma, does accumulate quite appreciable quantities of 

 iodine in the endostyle (Thomas, 1956), but this is associated with 

 mucopolysaccharides, and not with glycoproteins, as it is in 

 thyroid glands. There is no evidence of any action of the iodine 

 compound upon the amphioxus itself, though extracts provide 

 iodine that can be utilized by the thyroid glands of Amphibia 

 (Harrington, 1958). 



Agnatha. Lampreys undergo a well-defined metamorphosis 



