I 



OF THE NEMATOIDS, PAEASITIC AND FREE. 573 



doubt that the four papillae on the cephalic lobes of the Ascarides are also sense- 

 organs of some sort, cither simply tactile in their nature, or perchance endowed with 

 the faculty of recognizing qualities in bodies different from those which we are capable 

 of appreciating by any of our five senses. No nerve-filaments have yet been traced to 

 either kind of papillae, and whether or not they have any suctorial properties seems a 

 matter of uncertainty. Concerning what Schneider has termed the " cervical and caudal 

 papillae " in the lateral regions of the body, I have already spoken. It seems to me by 

 no means unlikely that the head-lobes themselves in the Ascarides are principally tactile 

 appendages, a very large quantity of nerve-fibres pass into them, and in their substance 

 they show a reticulated network of fibres of some kind (Plate XXIV. Z, I, m), which I 

 suspect are in great part nervous. 



ORGANS OF DIGESTION. 



The alimentary canal in the Nematoids is usually a simple, unconvoluted tube, extending 

 through the body in the parasitic species from the terminal mouth to an anus also ter- 

 minal, or situated but a very short distance from the posterior extremity of the body. 

 In many of the free Nematoids, however, the posterior extremity of the body extends for 

 a considerable distance behind the anus. This tube is always divisible into two distinct 

 portions, an anterior oesophageal part (separated from the next by a constriction), which 

 may be either simple or provided with one or more swellings in its course, and ha\ing 

 walls either strongly muscular or for the most part cellular; and a posterior part or 

 intestine proper, with no appreciable muscular tissue in its walls, but always having a 

 more or less developed cellular sheath, performing probably an hepatic function. The 

 length of the oesophagus, as compared with that of the body of the animal, varies much 

 in different species, though as a general rule it is proportionally longer in the free than 

 in the parasitic species. In the former it frequently occupies one-sixth or one-fifth of 

 the whole length of the body, and in Splicerolaimiis hirsutu^ as much as one-third, whilst 

 in the parasitic species it frequently does not occupy more than from ^ to -^ of the length 

 of the body, and in many species of Filaria even less than this. There are exceptions, 

 however, to this rule met with in the genera TricJiocephalus and Trichosoma, which are 

 most notable for the length of the oesophageal portion of the body ; in this respect, as 

 well as in the structure of their oesophagus and_ many other points in their anatomy, 

 these animals show decided affinities to the free Nematoids, although in some other 

 respects they are widely different. 



The oral opening in the Nematoids seems most frequently to be triradiate, as it is in 

 the members of the genus Ascaris, though in some species of the parasitic and in many 

 of the free animals it appears to be circular. So far as I have seen, it is always terminal 

 and situated in the centre of the anterior extremity. 



A pharyngeal cavity is rather the exception than the rule, though it exists to a well- 

 marked degree amongst the parasitic Nematoids in the genera Cucnllanm, Sclei'0st07mtmy 



