243 
NEOECHINORHYNCHUS CRASSUS, Nn. sp. 
(Pl. XXVI, Fig. 24, 25, 28) 
Body short and thick, almost cylindrical, tapering but slightly toward 
either extremity. Observed males 4 to 7 mm. long; females 6 to 9 mm. 
Maximum diameter of body usually in region of second dorsal sub- 
cuticular nucleus, just behind the single mid-ventral nucleus; in males 
usually slightly more than one-tenth of the total body-length, in females 
slightly less than one-tenth of the same. Body wall, especially the sub- 
cuticula, very thick, usually from 80 to 100 » except in certain regions in 
anterior part of body of gravid females, where it becomes considerably 
thinner, frequently reaching only about 60 ». Proboscis 0.27 to 0.325 mm. 
long and 0.24 to 0.27 mm. in diameter. Armed with hres circles of six 
hooks each. Hooks of terminal circle only provided with prominent roots. 
Terminal hooks 94 to 100 » long; hooks of middle circle 71 to 83 »; those 
of basal circle 47 to 71 p. Proboscis receptacle typical of the genus in 
shape and structure; 0.45 to 0.6 mm. in length. Testes in largest males 
approximately the same size, 0.87 by 0.38 mm.; in broad contact with 
each other. In smaller males the anterior testis is the larger. Cement 
gland, in structure, typical of that described for the family; crowded 
into hind margin of posterior testis; approximately the same size as 
posterior testis except in largest specimens, in which it reaches 1.25 by 
0.4 mm.; contains eight giant nuclei. Hard-shelled embryos within body 
of gravid female 35 by 14 yp. 
Cotypes in collection of U. S. National Museum and in collections 
of G. R. La Rue and of H. J. Van Cleave. 
Host, Catostomus commersoni (Lacép.). 
Type locality, Douglas Lake, Michigan. 
This species in many respects resembles the greatly variable Medi- 
terranean species, N.agilis. The two species are, however, easily sepa- 
rated on the basis of general appearance even though the measurements 
and data usually given in specific definitions do closely agree. Biological 
evidence and morphological data taken together, give sufficient grounds 
for the ready differentiation of the two species. There is fairly strong 
evidence that NV. agilis does not occur outside the Mediterranean, where it 
is found in fishes of the genus Mugil. Though fishes of this genus occur 
on the Atlantic coast of North America they have never been found to 
harbor any Acanthocephala. It seems improbable that a given species of 
Acanthocephala, N. agilis, for example, could have been brought to this 
continent by a marine fish and become established in an inland lake, 
leaving no trace of its transition from a marine to a fresh-water form. 
Numerous minor differences in structure give sufficient evidence of the 
distinctness of the two species even though ranges of variability in meas- 
urements for the two species frequently overlap. In general body-shape 
N.crassus is nearly cylindrical with a sudden diminution in size at each 
extremity, while N. agilis shows a conspicuous gradual tapering in both 
directions from the region of maximum diameter. The posterior two 
