1905-6.] Echinorhynchus antarcticus and its Allies. 
439 
body-region bears spines of the same type as those occurring in the 
anterior part, but upon a restricted area only. The shape of the 
parasite is such that, when attached by the rostrum to the aliment- 
ary canal of the host, one side will be applied along its whole 
length to the intestinal or stomach-wall. This side, together with 
a very short portion encircling the tip, constitutes the spiny area ; 
the whole remaining surface of the posterior region is quite naked. 
Since the object of the spines is primarily to assist in maintaining 
secure fixation, the significance of their one-sided distribution in 
an animal of this peculiar shape is apparent. Their occurrence 
around the tip (genital area) suggests a possible use in copulation. 
External Sex Characters. 
There is well-marked sexual dimorphism. This is not un- 
common in the Acanthocephala. According to de Marval “ les 
Acanthocephales presentent un dimorphisme sexuel souvent tres 
accentue, et qu’en these generale, les femelles sont beaucoup plus 
grandes que le males, voire meme quelquefois geantes.” The 
present species exhibits the unusual peculiarity that the males are 
larger than the females. In support of this interesting fact, 
which is an exception to what generally obtains amongst animals, 
it seems worth while to quote the following measurements : — 
Length of 1 2 Males 
examined. 
Length of 32 Females examined. 
mm. 
mm. 
mm. 
mm. 
5’25 
3*5 
3-75 
3‘5 
4*20 
4-2 
4-0 
4-0 
5*20 
4-0 
3-6 
4-1 
4*20 
4-0 
35 
3*0 
4*10 
3-5 
3-6 
4-0 
5-00 
3-5 
4-1 
4*1 
4-00 
3*8 
3-8 
3*2 
3-80 
3*8 
3-4 
3-5 
4-30 
3*5 
3-9 
4-75 
4*3 
3*2 
45 
3*4 
3-75 
4-2 
3-8 
4-2 
