CESTODA. 107 



for amongst the fishes, molluscs, and other aquatic animals on 

 which these feathered tribes feed. 



Reptiles. — As before hinted the cestodes are notoriously scarce 

 in all the reptilian vertebrates, even in the salamanders and batra- 

 chians, where many other kinds of entozoa are uniformly present. 

 Not a single cestode occurs either in the common or in the escu- 

 lent frog, and only one species has been detected in the toad. 

 IN'one are yet known to infest the crocodiles and tortoises, but one 

 species has been found in the common turtle. The cestodes are 

 almost entirely unrepresented in the lizards, and only a few forms 

 have been discovered in the curious amphisbsenge and ophidians. 



Fishes. — A great variety both of adult and larval cestodes take 

 up their residence in piscine "hosts." Usually such tapeworms 

 display characters very distinctive from those inhabiting birds and 

 mammals, being commonly furnished with special tentacular hook- 

 appendages employed as supplementary organs of boring and 

 anchorage. In the cartilaginous sharks and rays these cestodes 

 are remarkably abundant, and in certain osseous species they are 

 scarcely less frequent. The only noteworthy kinds of fish which, 

 so far as our present knowledge extends, seem to be free from the 

 invasion of tapeworms are the sturgeons, blennies, gobios, mullets, 

 sparoids, and ScieenidaB. 



Comparatively few of the invertebrata appear to harbour the 

 cestode entozoa in their adult condition, but to this rule the cuttle- 

 fishes form a noted exception. These singular animals also harbour 

 a great variety of tapeworm-larv^, forming one of the chief sources 

 whence the sharks and rays obtain the same parasites destined to 

 arrive at sexual maturity within their own bodies. Probably no 

 inconsiderable number of tapeworm-larvse abound in aquatic mol- 

 luscs, insects, and other invertebrate groups, but at present we are 

 only acquainted with a few distinct forms, the majority of which 

 are entertained by jelly-fishes. 



Nuviher. — It is almost as difficult to estimate the numbei' of 

 true cestode species at ])rescnt in existence, as it is to compute 



