No. 404.] MORTH-AMERICAN INVERTEBRATES. 647 
ORDER. — TREMATODA RUD. 
Small parasitic flatworms, with unsegmented, flattened or cylindrical, 
unciliated bodies, with usually anterior mouth-opening and bifurcate intes- 
tine, and without anal opening, which attach themselves to their hosts by 
. means of suckers, or hooks, or both. 
KEY TO THE SUBORDERS. 
A, Usually ectoparasitic trematodes living upon the external surface or 
the gills, or in the mouth or cloaca of aquatic animals (except genus 
Polystoma), to which they attach themselves by means of suckers, 
or hooks, or both; suckers, when present, are usually near either 
one or both ends of the body ; when at the anterior end, in most 
cases, a single pair is present ; when at the posterior end, in most 
cases, one or more pairs are present, or, in their place, a sucking 
disk . Heterocotylea Mont. 
A,. E adopri inikas which attach chamnasives to their hosts either 
y means of one or more median (unpaired) suckers or a large 
ventral sucking disk ; hooks never present. 
B, Either a large ventral sucking disk or a mid-ventral row of suck- 
ers present; no oral sucker; intestine not bifurcate (except 
possibly Aspidocotyle) . . . . . Aspidocotylea Mont. 
B, Either one or two or, in a few cases, more than two median 
Suckers present; an oral sucker invariably present (except 
Gasterostomum) ; intestine, except in rare cases, bifurcate 
Malacocotylea Mont. 
KEY TO THE FAMILIES, SUBFAMILIES, AND GENERA OF THE 
HETEROCOTYLEA. 
4;. Body with four to twelve finger-like tentacles at anterior end or on 
lateral sides, and with a posterior sucker ; lives on the outer surface 
of tropical fresh-water crustaceans and turtles, and in the branchial 
cavity of a mollusk ae . . Family I. Temnocephali 
6,. Tentacles all preoral . . Subfamily I. Temnocephaline 
cı Tentacles four to six in Tiada sucker subterminal; two pairs 
testes. 
@,. Dorsal'surface without lamellæ ; pharynx distinct 
Temnocephala Blanch. (Fig. 1) 
d,. pna surface with several transverse lamelle ; pharynx 
mentary . . . Craspedella Hasw. (Fig. 2) 
Ci Fania twelve in haia sucker very small ; one pair lobed 
testes ; pharynx distinct. . . . Dactylocephala Mont. 
^, Tentacles not all preoral . . Subfamily II. Actinodactynelline 
Tentacles twelve in number at sides and anterior end of pear-shaped 
