THE BIOGENETIC LAW 167 



tlien undergoes larval metamorphosis before it becomes a perfect 

 animal (Fig. iii). 



Fritz Miiller concluded from the repetition of the nauplius form 

 in all orders of Crustaceans that the primitive form of the Crustacean 

 must have been a nauplius, and that from it all the modern Crustaceans 

 must have evolved phyletically by the addition of segments varying in 

 number and ditferentiation. Now, however, it is doubted whether 

 there ever were nauplioid types capable of reproduction. But even if 

 the nauplii only represent tvhat have been the larval forms from very 

 early times, they are equally important in illustrating the relations 

 between ontogeny and phylogeny ; they at any rate represent the 

 primitive pre-cambrian larval form from which all modern Crustaceans 

 are derived. This is borne out not only by the facts to which we have 

 already referred, but also by those Crustacean-groups which have 

 diverged far from the usual Crustacean habit and type. 



Thus the sessile Cirrhipedes, with their mollusc-like shells, their 

 soft, unsegmented bodies, degenerate 

 heads, and their twelve vibratile food- 

 wafting limbs, emerge from the egg as 

 nauplius larvae. But the remarkable 

 parasites on the shore-crabs and the 

 hermit-crab deviate much further from 

 the type of the rest of the Crustaceans, 



for they hang like a sac or formless 



Ti (,. / ii 11 Fig. III. Nauplius larva from the 



SaUSage-hke soft mass to the abdomen ^^j^^^j. ^gg ^f lepMora hyalina ; after 



of their host, growing into it by ^ars. 

 tine, pale, root-like threads, through 



which they suck up the blood of their hosts (Fig. ii3, C. Sacc). They 

 possess neither head, nor thorax, nor abdomen, not even an indication 

 of segmentation, no limbs of any kind, neither antennae, nor mouth 

 parts, nor swimming-legs. Nevertheless they are Crustaceans ; indeed, 

 we can say with certainty that they belong to the order of Cirrhi- 

 pedes, for they leave the egg in the form of a nauplius larva {A)y 

 with ' horns ' on their carapace which no other forms except them- 

 selves and the Cirrhipedes possess. That they are of the same stock 

 as these is also proved by their further development, for the nauplius 

 grows first, just as in the case of the Cirrhipedes proper, into a ' Cypris- 

 like larva ' (B), so called because it bears a certain resemblance to the 

 Ostracods of the genus Cypris, and only from this point do their paths 

 of development diverge. The Cypris-like larva of the true Cirrhipedes 

 settles down somewhere, attached by its antennae ; it grows, and its 

 body becomes that of the perfect Cirrhipede; but the Cypris-like 



