THE DUCTLESS GLANDS 399 



as a chemical mechanism of integration relying on the trans- 

 portation of the stimulus (the secretions) through the vascular 

 system; and this mechanism is complementary to that of nervous 

 correlation and integration which involves not transportation 

 of stimuli but conduction of impulses arising from stimuli 

 along special paths, the nerves. " Secretin,' ' which is produced 

 by the lining of the intestine and stimulates the pancreas to 

 secrete, has been mentioned in Chapter XXVI. 



The Thyroid. — The thyroid was originally a longitudinal 

 tract of ciliated and mucous-producing cells on the floor of 

 the pharynx, called the endostyle. The endostyle is typically 

 represented in Amphioxus (and in the Ascidians), where it is 

 correlated with the ciliary method of feeding, and serves to 

 make a moving " fly paper," on to which particles of food 

 adhere and get carried safely back into the intestine (along the 

 hyperpharyngeal groove), instead of getting carried out through 

 the gill-slits by the outgoing current of water and lost. Such 

 an endostyle is also present in the Ammoccete larva of 

 Petromyzon. In the adult, however, it becomes closed off from 

 the pharynx and sunk beneath it, and it gives rise to the vesicles 

 of the thyroid. In all Gnathostomes the thyroid arises in 

 development from the floor of the pharynx, and in some 

 Selachii its cells still show traces of flagella. In the bony 

 fish, the thyroid is not enclosed in a capsule of connective 

 tissue, with the result that when it undergoes abnormal 

 growth (goitre) it may become carcinomatous and give rise 

 to a malignant cancer which invades the neighbouring tissues, 

 including the bones. In the higher forms the thyroid is 

 enclosed in a capsule. 



The secretion of the thyroid increases the speed of the 

 processes of metabolism in the body, and it has been said that 

 it stands in the same relation to the body as the draught does 

 to the fire. It plays an important part in the metamorphosis 

 of amphibia, by promoting the growth of the (previously 

 invisibly determined) regions into the organs which distinguish 

 the tadpole from the adult frog or newt. 



The Pituitary. — In all Craniates, the pituitary body is a 

 composite organ formed from the hypophysis which grows in 



