THE TREMATODA 49 



showed that it is an Annelid. While Kolliker pointed out that 

 Cuvier's " Hectocotyle " is not a fluke, but at the same time fell 

 into error in regarding it as a " pygmy male " of Argonauta and 

 Tremoctopus. Pentastoma was included, till P, J. van Beneden 

 (1849) discovered its embryo, and allocated it to the Arthropoda. 



The first definite attempt to classify the parasitic worms or 

 " Helminths," as they were then called, was made by Zeder (1800), 

 who divided them into five families, to which he gave German 

 equivalents of (a) round worms, (b) hooked worms, (c) sucking 

 worms, (d) tape-worms, and (e) bladder-worms. Of the " sucking 

 worms " he recognised three genera, of which he gave diagnoses, 

 and divided the various species into groups. Rudolphi (1808), in 

 an epoch-making work on intestinal worms, invented the term 

 " Trematoda " l for Zeder's " sucking worms," which he raised to 

 the rank of an ^ Order." 



Both these authors laid great and deserved stress on the 

 arrangement of the suckers, a character which, together with the 

 absence of " segmentation," is still a sufficient mark of distinction 

 of the Trematodes from the Cestodes. 



The earlier authors were acquainted with endoparasitic forms 

 only, and the discovery of ectoparasitic Trematodes, and the 

 gradual increase in the number of genera and species, as well as a 

 mor correct knowledge of anatomy, due to the researches of v. 

 Baer, Nordmann, Nitzsch, Diesing, and many others, led Leuckart 

 (1856) to propose a division of the Trematodes into the two 

 "families": (1) Distomea, for endoparasitic forms with a meta- 

 morphosis ; and (2) Polystomea, for ectoparasitic forms which have 

 no metamorphosis. 



In the same year, Burmeister separated Aspidogaster from the 

 rest, and suggested a threefold division into (a) Malacobothrii (for 

 Distomids), (b) Pectobothrii (for Polystomids), and (c) Aspidobothrii 

 (for Aspidogaster). 



This system has been generally overlooked and obscured by 

 P. J. van Beneden's great work (1858) on the group, embracing as 

 it did not only an account of several new species, but also an 

 experimental investigation into the life-history of the endoparasitic 

 and ectoparasitic forms respectively; these researches led to the 

 recognition of the importance of these two modes of reproduction : 

 the direct or " monogenetic," and the indirect or " digenetic." And 

 until quite recently this twofold division held the field, till Monti- 

 celli (1892) proposed the threefold division, which is essentially 

 the same as Burmeister's. 



The class Trematoda is divided into the three orders, primarily 

 distinguished by the character of the suckers, viz. Heterocotylea, 

 Aspidocotylea, and Malacocotylea. 



pierced with holes. 



