13] LIFE HISTORY OF T REM ATODES— FAUST 13 



MORPHOLOGY OF TREMATODES 



INTRODUCTION 



The progress in the morphological and histological knowledge of larval 

 trematodes is wrapped up in the development of discriminate observation 

 and interpretation on the part of investigators. This has been aided in no 

 small degree by the use of better technic and by better optical equipment, but 

 it is for the most part the observer's own expectation that differences must 

 exist in larvae, and his determination to discover faithfully and accurately 

 such a differentiation of structure, that has brought about progress in this 

 line of investigation. No clearer conception of the change in point of view 

 can be obtained than by a contrast of the statement of La Valette, a worker in 

 the field six decades ago, with the expression of Charles Sedgwick Minot just 

 ten years ago. In his Symbolae ad Trematodum Evolutionis Historiam 

 La Valette (1855:34) recites: "nonnuUae Trematodum larvae tam exiguam 

 offerunt diflferentiam ut discrimina earum characteristica vix commonstari 

 queant. " Minot (1897 :928) voices the modern point of view in his declaration 

 that "it is not true that all embryos are alike; on the contrary they show class, 

 ordinal, and generic differences from one another." 



While the writer fully agrees with the idea that the most natural way of 

 correlating larval trematodes with the adult forms is by a knowledge of their 

 life histories, yet such a correlation is not always possible. Looss (1896) 

 probably had the cercaria of Schistosoma haematobium among some of the 

 furcocercariae that came under his observation, yet he was forced to admit 

 (p. 167) that "tous ces efforts ont ete, quant a la Bilharzia completement 

 negatifs." The writer has attacked this part of the problem with the idea 

 in mind that not only the fundaments of the adult trematode are found in the 

 mature cercaria, but that even the main descriptive features of the adult 

 trematode are already present, so that the worker can recognize the adult 

 in the larva. While it has been impossible to show species correlations between 

 larva and adult it has been found in the course of the investigation that the 

 larva shows clearly the family features that hitherto have been inferred only 

 by the "blunderbuss method" of life-history investigations. 



Probably none of the adult trematodes genetically related to the larvae 

 studied have been described. Moreover, the characters common both to 

 larva and adult have been overlooked in the study of many adult species. 

 The writer has been confronted with the problem as to what characters of the 

 larva are ephemeral and what ones are common to cercaria and adult trematode. 

 A thoro analysis of the groups studied, including Monostomata, Holostomata, 

 and Distomata, gives convincing proof that the most constant systems in larva 

 and adult are the nervous, genital and excretory systems. Such systems and 

 organs as tail, cystogenous glands, and stylet are distinctly larval in nature 

 and may or may not show the same relationship as the natural grouping based 

 on characters common to both larva and adult. 



