Respiration in Invertebrate Animals. 253 



their functions respectively are alleged to be distinct and inde- 

 pendent. In what conceivable manner is the descriptive anato- 

 mist to depict the breathing systems of these animals, unless by 

 that of first adjusting these long-controverted questions ? The 

 idtimate structure of those solid parts, on which the office of 

 aerating the vital fluids is represented to devolve, must be first 

 determined. This inquiry alone can prove to what extent, if at 

 all, these parts are capable of answering the purpose which they 

 are stated to fulfil. The chylaqueous system of fluids exists in 

 every Echinoderm ; the water-vascular system does not exist in 

 every species. In the Sipuncles and the Ophiurida, it has no 

 place. The blood- vascular system is very imperfectly known. 

 Little has been done to demonstrate its presence in the asteroid 

 Echinoderms, and still less in the Echinidae. Its history has 

 been most fully developed in the Holothuridan and Sipunculidan 

 genera. 



1. The Chylaqueotis System of the Echinoderms. 



Is it capable of subserving a respiratory purpose ? Is it con- 

 stituted such that it is physiologically capable of executing this 

 great function ? And is it also distributed appropriately ? 



The mass of fluid occupying the visceral cavity, bounded on 

 one side by the digestive system, on the other by the integuments, 

 has been described, by the classical authorities upon this subject, 

 as consisting purely of sea water, admitted directly from without 

 through the skin, for the exclusive purpose of aerating the blood- 

 proper, said to circulate in a capillary system of vessels wrought 

 in the sohd parietes circumscribing the cavity. This, in succinct 

 expression, is the doctrine of the schools, as to the mechanism of 

 respiration in this interesting class of animals. It supposes the 

 existence of a profuse plexus of capillary vessels carrying true- 

 blood, distributed over all the visceral and parietal surfaces 

 limiting the chamber in question. It may be at once stated, that 

 no approach to a demonstration of the presence of this system 

 has ever been made by any modern or ancient anatomist. Is it 

 logical to erect one hypothesis upon another ? Let facts be first 

 represented. In the Asteridse, Echinidse, Ophiuridse and Ophio- 

 comidse, the fluid contained in the peritoneal cavity has been 

 described by every comparative anatomist as pure unmixed sea 

 water. It cannot be denied that the cavity itself is the anato- 

 mical homologue of the real perigastric chamber of zoophytes 

 and of the gastro-vascular canals of Medusae. It is therefore 

 the anatomical locale, in which the chylaqueous fluid should 

 accumulate; but under what character does it occur in the 

 higher vermiform Echinoderms? In the Holothuridan and 

 Sipunculidan genera (fig. 10, n), it presents itself as a chamber 



