24. Prof, J. C. Schiddte on the Mouth of Sucking Crustacea. 
15. When leaving the pouch the young of Cymothoa cestrum 
have a sharp-edged forehead, well developed, oval, prominent 
black eyes, slender, setiform antenne, the posterior pair so long 
that they reach as far as the middle of the tail, and slender 
limbs with long claws, which are hooked only at the point, and 
which, on those three pairs of legs which are directed forwards, 
assume the form of saws, owing to a row of powerful teeth on 
the underside. The tail is entirely free, not much shorter than 
the trunk, conical, its rings being very freely moveable. Its 
limbs possess long downy swimming-bristles ; the last pair are 
almost as long as the first fiye rings of the tail, and point 
straight backwards, their branches being elongated, narrow, 
with long downy swimming-hairs at the end. The seventh 
ee of limbs are wanting as in other newborn young of 
sopoda. 
ee to the classification hitherto current, these young 
Crustacea would rather be allied to Crrolana than to Cymothoa ; 
but the facts demonstrated in the foregoing investigation needed 
scarcely this addition in order to prove that here, too, in our 
attempts at system we go astray in darkness when we neglect 
the light afforded by the structure of the mouth. Ifthe young 
of Cymothoa, in the form of body, antenne, and legs, more 
reminds one of Cirolana than of the adult Cymothoa, and if 
an Aga has less external similarity to Cymothoa than to Ciro- 
lana, then all these similarities belong entirely to the class of 
biological modifications, and are without any typical character 
at all. Itis by marks of distinction of the same kind that 
Bopyrt have been separated from the other Isopoda ; but as their 
mouth is of the same construction as in Cymothoa, only far more 
reduced, they ought to be united with Cymothoa, diga, and 
their related genera into one natural family—Cymothoe. Not 
even in the characters of distinction now in use are transitions 
wanting; for there exist twisted species (not yet described) 
of the family of Cymothor, according to the definition of 
the family hitherto accepted, but which lack the last pair 
of caudal limbs. Upon the whole we may observe a striking 
parallelism between this present series of parasites on the one 
side and Siphonostomata on the other side, although it is not 
so extensive nor descends so low as the latter, at least accord- 
ing to our present knowledge. ga corresponds to Caligus, 
Cymothoa to Lernanthropus, Bopyrus to Chondracanthus. It 
is the greater or smaller degree of locomotion which decides 
the shape of the frontal margin, In those parasites which are 
continually fixed it is blunted ; in those which move about it is 
sharpened by the addition of the basal joint of the first pair of 
antenne. In Aga these antenne are still tolerably free, but 
