62 ILLINOIS BIOLOGICAL MONOGRAPHS [182 



Later development 



The later development in both species consists of uninterrupted growth 

 and differentiation from the time the first rudiments of the organs of the 

 adult appear to the time the parasites are ready to leave their hosts. 



The question of what constitutes a larval stage in the Gordiacea has 

 become even more complicated thru the present studies. Villot regarded 

 the larva as an embryo and designated the parasitic stage as larva. Nearly 

 all other writers have considered the stage that is free living after leaving 

 the egg as larva and the later stages as developmental, young or juvenile. 

 The application of the name larva to the parasitic stage was regarded as 

 incorrect because it had no remnant of the earlier larva except the degene- 

 rated proboscis and there was no definite change that marked the transi- 

 tion between this stage and the adult. The discovery of the larval cuticula 

 and the fact that it is shed at the time the parasite becomes ready to leave 

 its host removes to a great extent the objections to the application of the 

 term to the parasitic stage, and the term larva used to designate all the 

 stages from the time the free living form emerges from the egg to the time 

 the parasite is ready to leave the host would certainly be justified. But 

 since the free living and the parasitic stages are in many ways completely 

 different, I have used the term larva in this paper to designate only the 

 free living form and have used for the other the term parasite or parasitic 

 stage. The term embryo is incorrectly used when applied to the free living 

 form. The other terms for the parasitic stage have for the most part been 

 avoided in this paper because they are misleading. The term juvenile can 

 be applied to any other stage except the adult. The term developmental 

 can just as correctly be applied to the embryological stage as to the para- 

 sitic. 



Cuticula. During the entire developmental period the larval cuticula 

 expands and also increases in thickness. In that respect it differs from the 

 cuticula of arthropods. 



The fibrous cuticula of the adult is in neither of the species a secretion 

 of the hypoderm, but a differentiation of the upper parts of the cells, as 

 was already pointed out by Rauther. In other respects the cuticula in the 

 two species is formed very differently. In Gordius robustus there is an 

 intermediate layer formed between larval and the adult cuticula, the non- 

 fibrous cuticula is laid down before the fibrous cuticula begins to be formed, 

 the bristles are projections of the fibrous cuticula, and there are no evident 

 protoplasmic connections between the non-fibrous cuticula and the hypo- 

 derm. In Paragordius varius the larval cuticula remains in contact with 

 the non-fibrous cuticula, the structures of the non-fibrous cuticula are 

 laid down by protoplasmic strands that extend up to it from the hypoderm 

 and are present even in the adult, and the bristles or tubercles are struc- 

 tures of the non-fibrous cuticula. The larger radiating fibers that form the 



