OF THE NEMATOIDS, PARASITIC AND FREE. 559 



In many of the free Nematoids peculiar circular markings are met with close behind 

 the head, on either side of the body ; in others, compressed spiral instead of circular 

 markings exist; whilst in some, circular depressions or convex projections are substi- 

 tuted for these markings, in the same region *. I have examined these structures most 

 carefully with one of Powell and Lealand's ■^"' object-glasses, but could never detect 

 any aperture or internal vessel in communication with them ; they seemed to be simple 

 markings, elevations, or depressions of the integument unconnected with any other struc- 

 ture. In the members of the genus Plectus, however, somewhat similar markings exist, 

 which are undoubtedly connected either by a minute pore or by a thinning of the inte- 

 gument in this situation, with the anterior extremity of a lateral vessel (Plate XXVIII. 

 ftg. 14). 



It seems now to be an undoubted fact that, during the period of growth of the Nema- 

 toids, both free and parasitic, a periodical shedding and renewal of the chitinous portion 

 of their integument takes place. This was I believe first recorded by EHEEifBERG as 

 occuning ^amongst the free Nematoids. Amongst these I also have seen animals 

 belonging to several genera with the old integument separated from, though still 

 more or less investing the animal, as well as at an earlier stage, when the demarcation 

 between the old and the new chitine is indicated by a sharply defined undulating line. 

 In some species of the genera Oncholaimus and Cyatholaimus it seems to separate in 

 distinct shred-like pieces instead of being thrown off entire. In the genus Enoplus 

 the three teeth are shed and renewed with the integument. Amongst the parasitic 

 species I have seen a specimen of the so-called Filaria pisdtcm completely shed this same 

 chitinous portion of its integument, which phenomenon has also been described by 

 DuJAEDiN as occurring in HystricMs tricolor, and by Diesing in Ascaris depressa. 



MUSCULAR SYSTEM. 

 Almost all Nematoids seem to possess four great longitudinal muscles running along 

 the whole length of the body, and separated from one another only by the lateral and 

 median lines. In the species, however, in which the median lines do not exist, there is 

 often no distinct separation between the two dorsal or the two ventral muscles, and ac- 

 cordingly it has been asserted by some that two great longitudinal muscles only exist, an 

 upper and a lower. With regard to the presence or absence and distribution of trans- 

 verse fibres, much difference of opinion has existed. CLOQUEif has described transverse 

 muscular fibres external to the longitudinal in Ascaris lumhricoides, and OwenJ the 

 same in Strongylus gigas ; both speak of the close adhesion of these to the chitinous 

 integument. De Blainville§ speaks of transverse fibres internal to the longitudinal, 

 and Von Siebold || doing the same, correctly points out that these do not form com- 

 plete rings, but are as it were separated into four segments by the longitudinal lines. 



• See figures, loc. dt. f Sur les Vers Inteetinaux, 1824, p. 17. 



J Cyclop, of Anat. and Phys., Art. Entozoa, p. 129. § Diet, des Sc. Nat. torn. iiL App. p. 40. 



II Manuel d'Anat. Comp. note, p. 119. 



