XIX] EXCRETORY SYSTEM 439 



1. The pronephrio chamber produced by the fusion of several nephrotomes. 

 I 1 . The pronephric tubules. 2. The Malpighian capsules of the 



meson^phros. 2 1 . The primary mesoaephric tubules. 2 2 . The 



secondary mesonephric tubules. 3. The peritoneal funnels of the 



mesonephros. 4. The vasa efferentia and the longitudinal duct which 



unites them. 5. The Malpighian capsules of the metanephros. 



5 1 . The ureter. 6. The archinephric duct. 7. Young genital 



organ. 8. Ovary. 9. Testes. 10. Oviduct. 11. Vestigial 

 oviduct iu male. 12. Rectum. 



derived the muscles by which the locomotion of the animal as a whole 

 is carried out. In the lower Craniata these have the same simple 

 arrangement as was found in the case of Amphioxus, but in the higher 

 forms where the movements are complicated by the development of 

 limbs, these muscles are divided into numerous bundles with a very 



.complex arrangement. All the muscles derived from the myo tomes 

 are composed of striated fibres. Most of those governing the move- 



'm'ents of the alimentary canal and. blood-vessels are composed of 

 smooth fibres, but to this statement the muscles of the heart form 

 an exception. These latter, the cardiac muscles as they are 

 named, are cross-striated, but they are nevertheless very different 

 from the cross-striated muscles derived from the myotomes. The 

 cardiac muscles are short, thick fibres clearly divided by transverse 

 septa into the cells to which they owe their origin. Each cell has 

 a single nucleus. Neighbouring fibres are connected together by 

 cytoplasmic bridges so that they contract together. Muscles derived 

 from the myotomes consist of long fibres, each the product of a 

 single cell but each containing numerous nuclei. These nuclei are 

 scattered throughout the substance of the fibre, each embedded in 

 a little islet of unmodified cytoplasm. 



The excretory and reproductive organs are closely related 

 in development, and by recent research their relation 

 enital to those of the Cephalochorda has been made 

 tolerably plain. The unit in the excretory system 

 is a tube opening into the body-cavity at one end and at the other 

 into a longitudinal duct which opens into the proctodaeum behind. 

 The section of the body-cavity into which it opens is termed the 

 n e p h r o t o m e. The nephrotome is really a portion of the segmented 

 division of the body-cavity lying immediately beneath the myotome 

 into which at first it open's ; it is also connected with the splanch- 

 nocoel below. 



The kidney tube is in fact an outgrowth of the nephrotome ; 

 the longitudinal duct, which is termed the archinephric duct seems 



