410 THE AMERICAN NATURALIST. [ VoL. XXXV. 
According to Looss the formation of a special genus is warranted 
when a certain definitely circumscribed complex of characters can be 
recognized in two forms which also agree in other respects ; yet 
genera may be founded on single forms of evidently isolated struc- 
ture. While general appearance is of value, yet internal anatomy is 
the real basis of subdivision, and just this is, in fact, little known, 
partly at least owing to ignorance regarding the relative importance 
of characters. Among the most weighty generic characteristics are 
the copulatory organs which show the following types : (1) No mus- 
cular cirrus sac closed proximally and distally about the duct and 
seminal vesicle, together with the constant, if often weakly developed, 
prostate which lies (2) free in the parenchyme, or (4) enclosed ina 
connective-tissue covering open at both ends ; (2) a closed muscular 
cirrus sac which encloses (a) the genital sinus, é.c., the more or less 
elongated common terminal portion of both male and female ducts, 
or (4) only the end of the male duct. Here again the cirrus sac may 
enclose (a) seminal vesicle, prostate, ejaculatory duct, and protru- 
sible cirrus, or (4) only the last three, the vesicle lying in the paren- 
chyme, or (c) the prostate also is free, while only the duct and the 
cirrus are enclosed in the sac. The course of the uterus in the body 
is also an important generic character, while the size of the eggs is 
uniform within narrow limits in any genus. Of specific value are the 
size and form of organs in detail, the extent of the vitellaria, a very 
constant feature in any species, and similar details. 
In the section treating of the characters of the subfamilies and 
genera one finds a great variety in manner of treatment. Most 
groups are considered in extenso with full-faced headings which claim 
immediate attention, but there are those which are introduced in the 
middle of a topic under another heading, or even rarely one finds a 
new genus thrown in parenthetically which, in the absence of key 
and index, makes its discovery difficult. Still the work is generally 
free from such slips, and the absence of a key is attributable to the 
often repeated assertion of the author that this is a fragment and not 
a finished system, having for its primary object the demonstration of 
the existence of natural groups of family and generic rank within the 
limits of the old genus Distomum Retzius. 
From this section, as well as from the fourth and last, which con- 
tains a description of the new and little known species that have 
been studied by the author, it is hopeless to give here anything 
regarding the wealth of descriptive and comparative matter which 3 
offered. Itis not too much to say that no other helminthologist 15 
