WORMS 193 



the vitamin B complex is withheld from the diet of the host the tape- 

 worm produces no eggs but increases in size. On the other hand, lack 

 of vitamin B2 (vitamin G of some authors) in the diet of female rats 

 (but, curiously enough, not in males) causes their tapeworms to remain 

 undersized and stunted. Absence of carbohydrates in the diet and 

 possibly castration of the host, also inhibits their growth. In the labora- 

 tory it is frequently found that the host does not "do well" in captivity 

 and then the worms likewise appear in poor condition. All this goes to 

 emphasise a rather obvious fact that the relationship which exists 

 between worms and their hosts is both complicated and profoundly 

 intimate. 



In one way, at any rate, cestodes are unique in the animal kingdom. 

 Their protein content is less than the sum of their glycogen and fat. 



The tapeworms are certainly a very ancient group and they have 

 probably been committed to parasitism far longer than any of the other 

 flat worms, but their origins are lost in obscurity. There are no free- 

 living species, and apart from the egg, all the larval stages are endo- 

 parasitic — "they know not the light." 



Birds must have undergone a great deal of their evolutionary 

 history accompanied by tapeworms as well as Mallophaga. They are, 

 therefore, a favourite subject with those systematists who try to demon- 

 strate the true relationship of the parasites and their hosts, by studying 

 them together. Krabbe first pointed out that each order of birds has 

 its own particular cestode fauna and that consequently these worms can 

 throw some light on the relationships of the birds. Over 900 cyclo- 

 phyllid tapeworms (ten families and 135 genera) are recorded from 

 avian hosts, and of the 45 known orders of birds 41 harbour " Taenias." 

 Baer has elaborated Krabbe's theory and obtained some interesting results 

 from his studies. Grebes and divers for instance harbour quite distinct 

 tapeworms and the fact strongly suggests that the two groups of birds 

 should not be placed together — as they often have been — in a natural 

 scheme of classification. Two genera, Schistotaenia and Tatria, are con- 

 fined entirely to grebes and, moreover, occur in this order all over the 

 world. The genera Gyrocelia and Progynotaenia are found in waders only. 

 Swallows and swifts each have distinct genera of tapeworms and 

 although they share Anomo taenia the species are different. This latter 

 genus is very large and spread through many orders, but certain species 

 are often characteristic of closely related species of birds which are 

 separated by wide geographical barriers. Thus, Anomotaenia constricta is 



