58 LECTURE IV. 



of the atmosphere, no distinct respiratory organ could be expected to 

 be developed in the Entozoa; but this negative character is common 

 to them with most of the other " Zoophytes " of Cuvier. In animals 

 surrounded by and having every part of their absorbent surface in con- 

 tact with the secreted and vitalised juices of higher animals, one might 

 likewise have anticipated little complexity and less variety of organ- 

 isation. Yet the workmanship of the Divine Artificer is sufficiently 

 complicated and marvellous in these outcasts, as they may be termed, 

 of the Animal kingdom, to exhaust the utmost skill and patience of 

 the anatomist in unravelling their structure, and the greatest acumen 

 and judgment of the physiologist in determining the functions and 

 analogies of the structures so discovered. What also is very re- 

 markable, the gradations of organisation that are traceable in the in- 

 ternal parasites reach extremes as remote, and connect those animals 

 by links as diversified, as in any of the other groups of Zoophytes, 

 although these play their parts in the open and diversified field of 

 Nature. 



Beginning with the lowest link of the Entozoal chain, we have to 

 commence with a condition of organisation more simple than is 

 presented by the lowest Infusory or Polype. We end with a grade 

 of organisation, which, whether it is to be referred to the radiated or 

 articulated types, zoologists and anatomists are not yet unanimous. 



Amongst the vermiform animals with colourless integument, colour- 

 less circulating juices and without respiratory organs, two leading dif- 

 ferences of the digestive system have been recognised : in the one it 

 is a tube with two apertures contained in a distinct abdominal cavity ; 

 in the other it is excavated or imbedded in the common parenchyme 

 of the body, and has no anal outlet. The first condition characterises 

 the Vers Intestinaux Cavitaires of Cuvier ; the second the Vers In- 

 testinaux Parenchymateux of the same naturalist.* 



I have rendered the Cuvierian definitions of the two leading 

 classes or groups of the Entozoa by the single-worded names "Coelel- 

 mintha," and " Sterelmintha." 



The Coelelmintha include the Linguatulce with the Gordiacea or 

 * hair-worms', and the cylindrical Entozoa or 'round worms' which 

 form the order Nematoidea of Rudolphi. 



This great entozoologist, who devoted the leisure of a long life 

 to the successful study of the present uninviting class, divided the 

 parenchymatous Entozoa, here associated in the class Sterelmintha^ 

 into four orders. The Acanthocephala, in which the head has a 

 retractile proboscis armed with recurved spines, the body round and 



* XII. torn. iv. p. 38. 



