134 Dr. T. Williams on the so-called "Water-vascular St/ stem" 



and Cestoid Entozoa, has no reference whatever to such a func- 

 tion; that in the latter animals the miscalled water-canals are 

 really nutritive channels ; that they are probably furnished with 

 a suctorial and an excretory extremity ; and that what has been 

 described as the ciliated "water-tubes^^ in the Trematodes do 

 not carry water at all, under any conditions or circumstances. 



3. That in the entire family of the Medusa there exists no 

 structure which is entitled to be styled a water- vascular system. 



4. That in the Echinodermata there is no system of vessels 

 which carries water, in the respiratory sense of that phrase ; that 

 the contents of the ambulacral system are not pure water, but 

 on the contrary a fluid drawn from the cavitary reservoir, and 

 destined to be eliminated externally, like the contents of the vas- 

 cular system of the Annelida ; that from the analogy of all the 

 facts now known, it is in the highest degree probable that the 

 " branchial tree" of Holothuria is not branchial at all ; and that 

 it does not suck-in water from without under any circumstances. 



5. That in the entire family of the Annelida no system of 

 organs can be discovered which deserves to be called a water- 

 vascular system. 



There can be no doubt that the hsematosine and metallic 

 compounds which accumulate in the vascular fluid system of 

 this class prove that it is intended, where it is present, to fulfil, 

 in an express manner, a respiratory function, and therefore corre- 

 sponds with 



6. The tracheal system of Insects. 



But in the former instance the external aerating medium is 

 not admitted directly into the vessels, as in the latter. This 

 point is one of material difference. An aerial-vascular system 

 is thus proved to exist, but not a it^^^fer-vascular one. 



7. I have not been able to prove in any instance among the 

 Crustacea and INIoUusca that water is directly and immediately 

 admitted into any one of the systems of vessels or channels 

 communicating with the interior of the body. 



8. The preceding corollaries are founded upon a vast number 

 of practical details (which will be published at length in my 

 work on the Respiratory Organs), which have led me, step by 

 step, to the conclusion that a real " ivater -vascular system" has 

 no existence in any class of Invertebrated animals; and that 

 really there is not a single example amongst the Invertebrata 

 of that method of breathing which has been called "internal 

 aquatic respiration." 



I remain. Gentlemen, 



Your obedient Servant, 



Thomas Williams. 

 Swansea, Jan. 12, 1857. 



