22 INVEKTEBEATA CHAP. 



were really ancestral, what was being reproduced were primarily 

 adaptations to an ancestral environment. 



The proof that the embryonic stage is a concealed larval one exists 

 widespread in the animal kingdom. When we find that the Nauplius 

 stage of the shrimp Penaeus lives as a minute self-sustaining organism, 

 using the tiny hooks at the bases of its second and third appendages as 

 jaws to seize its prey ; and that the corresponding stage in the develop- 

 ment of the crayfish is passed within the egg-shell, but that the embryo 

 has the Nauplius limbs in the condition of useless stumps, although, 

 just as in the case of the free-swimming larva, the passage into the 

 next stage of embryonic life is initiated by a shedding of the cuticle, 

 then we have no doubt which is the more primitive, the larva or the 

 embryo. So too, when we find in the development of the Martinique 

 toad Hylodes that the embryo within the egg has the tail of a tad- 

 pole which is never used for swimming but is absorbed directly the 

 animal hatches, we have no difficulty in concluding that the original 

 condition of affairs was that in which the tadpole used its tail for 

 the purpose for which its shape is adapted. 



Now the phase preceding the attainment of the adult form is 

 always larval (it is often termed brephic or neanic), and this, 

 according to the biogenetic law, should represent the last stage 

 which the race has passed through before attaining its present 

 condition, and will therefore be, generally speaking, the least modified 

 stage in the life-history, since it is the most recently added to the 

 series. Is there, then, evidence that this stage is of ancestral 

 significance ? 



The answer to this question is that there is abundant evidence 

 of it, and a few instances of this may now be given. 



The Oyster (Ostrea), contrary to the custom of the majority of bi- 

 valves, lies on one side, and remains fixed thus through life ; but the 

 American species, at least in its so-called "brephic" stage, when it has 

 terminated its free-swimming existence, creeps about for a short time, 

 and possesses, like other bivalves, a " foot," which is totally wanting 

 to the adult oyster. 



Speaking broadly, when we examine the life -history of any 

 aberrant member of a well-defined group in the animal kingdom, 

 we find that in a late stage of its life-history it resembles the normal 

 member of the group to which it belongs. Portunion, an Isopod para- 

 sitic on Crustacea, as distorted out of all resemblance to an Isopod, 

 but when young it is an unmistakable Isopod, a trifle simplified in 

 structure. The Crinoid Antedon when adult is devoid of a stalk, 

 and swims by muscular movements of its arms ; but when it is 

 young it possesses a stalk like the vast majority of its congeners. 

 The Plaice, Pleuronectes, swims with one side down, and both eyes are 

 twisted on to the upper side ; yet the larva of this fish has both sides 

 symmetrically formed with the eyes in the normal position, like the 

 vast majority of Teleostei. This list could be extended indefinitely, 

 and if the animals named are rightly classified in the groups in which 



