568 STRUCTURE OF VERTEBRATA 



from the ccelom, and containing a mass of capillaries which project 

 into the cavity of the tubule ; and (3) a coiled tube in part excretory, 

 in part a conducting canal for the waste filtered from the blood. The 

 metanephric tubules have a quite similar structure, but the nephro- 

 stome is never present. 



In all Vertebrates the primitive nephridia open into a 

 pair of longitudinal ducts, developed like the nephridia as 

 special portions of the ccelom. These ducts open into the 

 end of the gut. According to their connections with the 

 nephridia these longitudinal ducts are called pronephric, 

 mesonephric, or metanephric ducts, and they are also called 

 segmental ducts. In Elasmobranch fishes a Miillerian 

 duct is separated oflF from in front backwards from the 

 longitudinal duct and forms the oviduct of the female, a 

 rudiment in the male. After the separation of the 

 Miillerian duct, the longitudinal duct (now called meso- 

 nephric or Wolffian) forms in the male the vas deferens 

 and also receives the tubes from the permanent kidney 

 (mesonephros). In the female the Wolffian duct has this 

 last function. In general it may be said that the original 

 longitudinal duct becomes the vas deferens in the male 

 Vertebrate, and that another duct — the Miillerian — whose 

 development is obscure except in Elasmobranchs, forms 

 the oviduct. The metanephric duct, developed in part 

 from the hinder end of the mesonephric duct, is the ureter 

 of the permanent kidney in Amniota. 



Internal secretion.— In the Vertebrates the different 

 parts and functions of the body are co-ordinated not only 

 nervously but chemically, by means of the specific '* hor- 

 mones " or " internal secretions," elaborated by the "endo- 

 crine " or ductless glands and carried by the blood. It is 

 not certain that any such system occurs in Invertebrates, 

 unless in some Crustacea and Annelids. 



Suprarenal bodies. — These are found in most Verte- 

 brates near the reproductive organs and kidneys. They 

 seem to increase in importance as we ascend the series. 

 Typically, each shows a distinction into a cortical and a 

 medullary zone. It is usually asserted that these two 

 areas have a different origin, the medullary region being 

 derived from the sympathetic nervous system, the cortex 

 from the coelomic epithelium. There is much evidence 

 (morphological and physiological) that the suprarenals of 



