24 Prof. J. C. Schiodte on the Mouth of Sucking Crustacea. 



15. When leaving the pouch the young of Cymotlioa oestrum 

 have a sharp-edged forehead, well developed, oval, prominent 

 black eyes, slender, setiform antenna, 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 trmik, conical, its rings being very freely moveable. Its 

 limbs possess long downy swimming-bristles ; the last pair are 

 almost as long as the first five rings of the tail, and point 

 straight backwards, their branches being elongated, narrow, 

 with long downy swimming-hairs at the end. The seventh 

 pair of limbs are wanting as in other newborn young of 

 Isopoda. 



According to the classification hitherto current, these young 

 Crustacea would rather be allied to Cirolana than to Cymotlioa ; 

 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 structm-e of the mouth. If the young 

 of Cymotlioa^ in the form of body, antennas, and legs, more 

 reminds one of Cirolana than of the adult Cymotlioa^ and if 

 an ^ga has less external similarity to Cymotlioa than to Ciro- 

 lana^ then all these similarities belong entirely to the class of 

 biological modifications, and are without any typical character 

 at all. It is by marks of distinction of the sa^me kind that 

 Bopyrih.a.\Q been separated from the other Isopoda ; but as their 

 mouth is of the same construction as in Cymotlioa^ only far more 

 reduced, they ought to be united with Cymotlioa^ -^O^i^ ^^^ 

 their related genera into one natural family — CymothoEe. 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 Cymothoas, 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. 2Ega corresponds to Caligus, 

 Cymotlioa to LernantJiropus, Bopyrus to Chondr acanthus. 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 

 antennte. In ^ga these antennse are still tolerably free, but 



