ON OUR PRESENT KNOWLEDGE OF THE CRUSTACEA. 199 



ovum of the Braehynra, that it appears doubtful if there be any distinction 

 between Protozosea and Zosea. 



Fritz-Miiller comprehends under the term Zosea all those brephaliQarvss) 

 that have two pairs of antennas. The oral appendages and the gnathopoda 

 present the latter in the form of swimming appendages. Having in view 

 the young of the Brachyura, Anomura and Macrura, as well as certain stages 

 in the development of the Stomapoda, whilst he could not include the young 

 Schizopoda with the six pairs of legs (Euphasia) which Claus considers must 

 be accepted as a zoaaa form. Claus considers that there is a highly im- 

 portant character excluded from this definition, — the stage of the develop- 

 ment of the pereion, or, as he terms it, the limbless central body (Gleid- 

 massenlosen Mittelleibes) in contrast with the pleon (Hinterleib) and its ap- 

 pendages This is, he says, just the characteristic of the zosea, which needs 

 explanation, and at the same time contains the key tor the comprehension 

 of the structure of the zoasa stage of the Malacostraca. It is necessary to 

 understand and explain the striking relation of the pereion that exists in 

 an immature condition, and from which sprout the five pairs of pereiopoda 

 between the cephalon, with its numerous well-developed appendages and 

 the well-formed but still limbless pleon. He says that almost in all forms 

 of brephalus (larva) the pereion is either completely suppressed as in the 

 Decapoda, or appears in the form of rudimentary somites, as in Schizo- 

 poda and Stomapoda. The pereiopoda are produced later than the ap- 

 pendages of the pleon. " Of course," he continues, " an exception must be 

 made for the zoaea of Penceus, from which the limbs of the pereion are pro- 

 duced previously to those of the pleon, with the exception of the two 

 lateral appendages of the tail, which as belonging to the sixth somite of 

 the pleon appears sooner, or at least about the same period, as those of the 

 pereion." 



In arriving at this conclusion Claus appears to have gathered his facts 

 from too circumscribed an area. Assuming his observations on the 

 development of Penceus to be correct, he has overlooked that of the 

 typical zoaea when it quits the ovum, as seen in Garcinus Mcenas, and that 

 of Stenorhyncus, Inachus and Maia, of the latter two of which he has 

 himself given figures that represent the pereiopoda advancing in develop- 

 ment anterior in degree to that of the pleopoda. Moreover, the • bre- 

 phalus (larva) of Homarus and Palinurus have the pereiopoda well ad- 

 vanced in formation previously to any evidence of the pleopoda being in 

 existence. Whilst others have them developed in a common ratio. 



The zoaea of Crustacea therefore may be defined as a brephalus (larva) 

 that has two pairs of antennas, the oral appendages and gnathopoda more 

 or less developed, but in which the pereiopoda and pleopoda are yet 

 absent or in an immature condition. 



This is the condition in which the brephalus quits the ovum as the zoaea 

 of the Brachyura, Anomura, and some Macrura. But in each there is a 

 persistent feature that distinguishes one form from that of the others, and 

 as far as my own observations have led me precludes their being con- 

 founded one with the other. 



The brephalus of the brachyura is a zocea (PI. VI., figs. 3 and 4), and the 

 most constant as to its general type of all the families of the class. With 

 the exception Gecarcinus, which quits the ovum in the Megalopa stage, I 

 am not aware of any other of the short-tailed crabs that is not hatched 

 in the zoaea condition. 



That of Carcinus mcenas, as our most common European species, may 



