FREE NEMATOIDS. 
175 
Thus we see another instance of the fact that the artificial 
barriers which zoologists establish between different groups of 
animals, are after all purely conventional, and to be taken only 
in a broad sense. Examined more closely, we see that, however 
convenient and necessary such classifications may be for facili- 
tating our knowledge of the animal kingdom, no such barriers and 
strict demarcations exist in Nature — differences are gradually 
smoothed down and resemblances increase between animals of 
different zoological families which may widely differ from one 
another in their extreme types. 
EXPLANATION OF PLATE.* 
Fig. 
•n 
1 . 
2 . 
3. 
4. 
V 
5. 
6 . 
7. 
8 . 
9. 
Dorylaimus Stagnalis, Dujard. Female, X 20. 
Ditto Male, X 20. 
Anterior extremity of same, more highly magnified, showing 
exsertile spear, with reserve spear in substance of oesophagus, 
X 150. 
Posterior branch of uterus and ovary of Plecttis pm’ietinus, Bast., 
showing single file of large ova in latter, X 300. 
Anterior extremity of Tylenchus tritici, Bast,, X 150. 
Anterior extremity of Leptosomatwn Jiguratum, Bast., X 60. 
Anterior extremity of female Phanoderma Cochsi^ Bast., X 75. 
Posterior extremity of same, X 75. 
Posterior extremity of male Phanoderma Cocksi, X 75. 
elsewhere shown, and the male parasite of this species also has never been 
met with. Indeed, the life-history of this species, when fully ascertained, 
will probably approach very closely to that of Ascaris nigrovenosa ; but at 
present the Guinea- worm is unknown in its free condition. And instead of 
a regular and necessary alternation existing between the free and parasitic 
states of this animal, it would seem more probable that such alternation 
only occurs occasionally as an accidental and extraordinary occurrence in 
the life-history of certain individuals. 
* For explanation of letters of reference to figures see Family Description, 
' p. 167. 
