430 Sidney I. Kurnhauser 



shape themselves to the enviroiiment in which the animal is placecl can 

 be used as an absolute criterion in Classification. 



Let US examine, for instance, the discussion over the position of 

 Hersilia, which is a typical semi-parasitic form. Hersilia apodiformis 

 was first described and named by Philipp: in 1839. The new genus, he 

 considered, would fall between the genera Peltidium and Sapphirina. 

 Later Kossmann ('75) described anew this same animal, giving it the 

 name Clausidium testudo, nov. gat., nov. sp., and placing it on account 

 of its body form near Artotrocjus and Asterocheres, genera found on Echino- 

 derms. The latter work called forth a contribution by Claus ('75) against 

 Kossmann's new genus and species and against the association of Hersilia 

 (Clausidium) with the Asterocheridae, which have sucking mouthparts. 

 Claus maintained that the mouthparts formed the most important 

 character for Classification and considered Hersilia (which has mouth- 

 parts intermediate between those suited for biting and those suited for 

 sucking: small reduced mandibles, and long labrum) as probably originat- 

 ing from the Peltiidae and assuming a form which mingied the characters 

 of the Corycaeidae and Bomolochidae. In 1893 Giesbrecht published a 

 consideration of the systematic standing of the HersiUidae, a family 

 which had been constructed by Canu, including the genera Hersilia {Clau- 

 sidium), Hersiliodes, Giardella and Nicothoe. Giesbrecht objected to 

 Canu's association of this family together with the XereicoÜdae into a 

 group separated from the Corycaeidae, Oncaeidae, and Lichomolgidae. 

 He (Giesbrecht '93) held that the HersiUidae had their nearest relatives 

 in the Oncaeidae on account of the sirailarity of the first and second 

 antennae, the mandibles and maxillae of these two families. 



The foregoing example is probably sufficient to illustrate the diversity 

 of opinions which may arise, as to the importance of various morphological 

 Units in taxonomy. Giesbrecht pointed out that, through a study of 

 the ontogeny of parasitic forms, a better idea as to their relationship 

 could often be obtained than from a comparison of adult structures. 

 Pesta ('09) discusses Giesbrecht's Suggestion and also considers it to 

 be of greatest taxonomic value to determine in which cyclopid stage the 

 characters of parasitism first appear. As regards the large group of semi- 

 parasites, messmates, commensals and free-swimmers with niouth parts 

 transitional between those adapted for biting and those adapted for 

 sucking, the group to which the Hersiliidae and the Corycaeidae beloiig, 

 Professor Chas. Wilson ('IIb, p. 310) says: "Much more work must 

 be done upon the ecology, morphology, and especially the ontogeny of 

 the various families before any lasting suggestions can be made in the 



