absence of the sheath in some species might be explained 

 on the basis of delayed molting in which the formation 

 of the membrane takes place in the vector. 



The development of Trichinella sj^ralis should be 

 considered briefly from the standpoint of sheath formation. 

 Schwartz (1918) has demonstrated that trichinae removed 

 from the alimentary canal of rats will undergo molting 

 outside of the host. He does not state the number of 

 molts undergone but implies that there are at least two 

 before the adult stage is reached. So far as we are 

 aware no one has attempted to determine the possibility 

 of molting occurring before encapsulation of the larvae. 

 Raffensperger (1918) found that trichina larvae were 

 not infective 15, 17 and 18 days after infection, but 

 were after 21 days. Our experiments showed that the 

 point of infectivity was between 18 and 20 days. Nolf 

 and Edney (1935) found the larvae infective after 17 

 days. Undou"Dtedly at this time some biological change 

 occurs in the parasite, probably associated with molting, 

 which results in their infectivity. The explanation of 

 some workers that infectivity is the result of encapsula- 

 tion will not hold since it is inconceivable that the con- 

 nective tissue capsule, which is dissolved in the stomach, 

 could offer any protection for the larvae against the 

 stomach barrier. It is more probable that infectivity 

 follows the second molt as is the general rule with other 

 nematodes. The presence of the shed cuticles in the 

 blood stream might be a contributory factor in producing 

 the eosinophilia associated with trichina infection. 



Fig. 138. 

 Eggs and embryos of Habronema innsrur. After Ransom, 1913, 

 U. S. D, A., B. A. I. Bull. No. 163. 



Special Morphology 



R. O. C. 



Eggs of parasitic nematodes are variously modified 

 to adapt them to ofttimes complicated life cycles. The 

 more obvious of these specializations are the presence of 

 byssi, terminal or polar filaments, equatorial filaments, 

 opercula, and the mammillation of the shell. Byssi (Fig. 

 139) are branched, polar cords forming tassel-like struc- 

 tures on the eggs of the Mermithoidea. Apparently 

 their function is to hold the eggs to the pubescent surfaces 

 of plants until they are eaten by the essential hosts. 

 Terminal filaments are found in certain genera of the 

 superfamilies Spiruroidea and Oxyuroidea. They are 

 unbranched structures which may occur singly, in pains, 

 or as tufts. These filaments may be unipolar or bipolar 

 in distribution. Foster (1914) advances the idea that 

 in Tctramcres (Pig. 141E) they function in holding the 

 eggs together to insure massive infection of the hosts. 

 Subpolar filaments are sometimes distributed over the 

 surface of the agg accompanying the polar tufts. Equator- 

 ial filaments (Fig. 135R) occur in the g-enus Pseudonynnis 

 of the superfamily Oxyuroidea. They may function in 

 the manner suggested by Foster tor the filaments of 

 Tetrameres. Chitwood (by correspondence) points out 

 that spe:ies with filaments on the eggs are basically asso- 

 ciated with an aquatic habitat, and that the filaments 

 may function by entangling the egg in the vegetation 



thus preventing it from settling into the debris of the 

 substratum which would reduce its chances of survival. 



Among free living nemas hooks (Anaplectus granulosus) 

 and minor excrescences {Rhabditis filiformis, MonoyicJnts 

 ptnictatiis, TrUobus pellucidus) of the protein layer occur 

 in some aquatic species. It is notable that Rhabditis 

 filiformis is one of the very few species of aquatic Rhab- 



Fig. 139. 



Eggs of ]\Ier>nis subnii/resrens showing variations in the form of 

 the byssus. After Christie, 1937. J. Agric. Res., v. 55 (5) : 353-364. 



Fig. 140. 



Egg of Mermis sulmigrescens. A — showing outer and inner layers 

 of shell ; B — with outer layer of shell removed, showing larva 

 within ; C — in process of hatching ; D — with larva emerging. After 

 Christie. 1937, J. Agric. Res., v. 55 (5) : 353-364. 



ditis and it is the only one known with a protein layer. 



Opercula are zones of escape by which the embryos 

 leave the egg membranes. They may have bipolar distri- 

 bution and appear plug-like as seen in the Trichuroidea, 

 and, to a lesser extent, in some of the other major groups. 

 Some nemas possess a single operculum, others an oper- 

 cular spot marked by the thinning of the membrane 

 in a certain region, or the shell in some species may 

 have lines of fracture indicating the area from which 

 the embryo leaves the egg. 



Mammillation, in the strict sense of the term, refers 

 to the rounded excrescences over the surface of the 



181 



