290 DK. T. S. COBBOLD ON STEONGTLUS TETEACANTHUS. 



Thus on the 27th of February, 1874, I received from Mr. W. 

 Cawthrou, V.S., of Hadlow, a box holding forty little "cysts or 

 bladders, each containing a worm." These cysts, as Mr. Cawthron 

 termed them, were removed by him from faeces passed byatwo-year 

 old colt, together with Oxyurides and large Ascarides, from the 

 presence of which the animal was sufferiug and rapidly "wasting 

 away." Microscopic examination eatisfied me that these so- 

 called cysts were really faecal pellets, the particles of which had 

 been collected and held together by exudation from the young 

 worms. The vegetable debris thus collected formed a tind of 

 cocoon whose walls were made chiefly of vegetable parenchyma, 

 as shown by plates of muriform and polygonal cells, chlorophyll, 

 spiral vessels, and even raphides. Vegetable hairs and fibres 

 also projected from the surface, here and there. Averaging the 

 size of ordinary pills, some of the cocoons displayed the heads 

 and tails of the enclosed worms standing out as finger-like pro- 

 cesses. Prom one of these coccoons the projecting tail of the 

 worm still retained a part of an old skin which the larva had not 

 fully cast. I have reproduced a small drawing of this singular 

 formation (fig. 15, Plate XXXYI.). In the history of the develop- 

 ment of the Nematoids I know of nothing comparable to this 

 cocoon-forming habit of Strongylus tetracantMis ; and I venture 

 to suggest that we have here a mechanical contrivance which 

 possibly supersedes the necessity of the parasites seeking entrance 

 to the body of an intermediary host. Be that as it may, the 

 cocoon serves as a protecting covering whilst the young worm 

 undergoes partial metamorphosis, attended with ecdysis. Inter- 

 nally, however, there were no visible traces of sexual differen- 

 tiation. In one worm I noticed minute prominences which 

 I supposed to be rudimentary cephalic spines. Within the 

 pellets a hollow tube corresponds with the form of the enclosed 

 larva, but I did not find any skin-cast within this cavity. 

 Whether my interpretation of the facts be correct or not, future 

 investigations must determine. A missing link, representing a 

 transition from the cocoon-stage to the young and unimpreg- 

 nated sexual worm, is still wanting to complete the chain of 

 evidence ; and it may turn out that a natural expulsion of the 

 cocoons by the host is a necessary provision to this end. The 

 immature females in the most advanced stage prior to impregna- 

 tion display large numbers of ovarian ova. I believe, however, 

 that all the final stages of growth occur within the bowel of the 



