196 A. S. PACKARD, JR., ON THE 



king crabs of to-day, and thus dispensed with the use of functional antennfe ; these organs 

 morphologically such, retaining the locomotive functions as in existing nauplii. A step higher 

 and we liave the Protocyclus, with short, ambulatory, cephalic limbs, the front part of the 

 body adapted still better than before for burrowing in the fine sand and mud, no antennse 

 being developed, while under the abdomen, the integument of the body having meanwhile 

 become harder, may have grown rudimentaiy, gill-bearing phyllopodous feet, like those of 

 the embryonic Limulus. With the gradually flattening form of the body, and small, swim- 

 ming feet, the Protocyclus may have adopted, at times, a more roving life, and thus the type 

 became widely distributed. In some such mode as this we should expect that a burrowing 

 Archicaris gradually assumed the Cyclus and Agnostus forms. On the other hand, the 

 ancestral Eurypterus must have swam more freely than the Protocyclus, since the append- 

 ages in this type representing the antenuEe are not only ambulatory organs, but also well 

 adapted for seizing and biting ; while the body lengthens out and the respiratory feet are 

 moved forwards from the telson, in unison wdth the requirements of more or less rapidly 

 moving predatory Crustaceans, as these forms undoubtedly must have been. 



The Nebaliadse are the most aberrant group of true Phyllopoda we have to deal with. 

 Metznikow, who has studied their embryology, concludes, as Miiller states, that Nebalia, 

 during its embryonal life, passes through the nauplius and zoea stages, which in the Deca- 

 poda occur partly (in Penaeus) in the free state. " Therefore I regard Nebalia as a phyllo- 

 podifovm Decapod." While I would not regard Nebalia with its eight pairs of well 

 developed thoracic phylliform feet, with the peculiar phyllopodous carapace adhering to the 

 head alone, as a Decapod, yet as remarked long ago by Milne Edwards, "Elles semblent, a 

 plusieurs egards, etablir le passage entre les Mysis et les Apus." The zoeal forms that 

 we see in the Phyllopoda and Eurypterida culminate in the embryonic zoea of Nebalia. 



On glancing at the diagram it will be seen that the Nebaliadse hold the centre, and pre- 

 sent an almost unbroken succession of forms from the lowest Silurian to the present day. 

 The true zoeal forms began with them. Their Lawrentian, nauplian ancestors must have 

 been free swimming forms, and in accordance with this habit, have by a continued roving 

 and more or less predatory life retained their sense organs or antennsB comparatively 

 unaltered, while after a series of nauplian moults, resulting in a fixed habit of metamor- 

 phosis, swimming respiratory feet were developed near the head, while the rudder-like 

 abdomen terminated in a bifid or trifid telson, and two pairs of small abdominal feet were 

 gradually developed. Thus we have comparatively highly organized metabolous Arthropoda 

 at the dawn of the Silurian age ; and if there is any truth in a development hypothesis, we 

 have the strongest reasons for supposing that they were preceded by simpler ametabolous 

 forms in an age anterior to the deposition of the lowest Silurian fossiliferous rocks, per- 

 haps the Huronian and even the upper Lawrentian period. ^ 



At any rate the earliest known Crustacea appear in highly developed forms representing 

 four (if we include the Merostomata) important groups of Entomostraca, and one, the most 



I'VVliile in the Insects, as Fritz Miiller has indicated, the while those with an incomplete metamorphosis (Trilobites, Me- 



ametabolous insects were the older, and the "complete meta- rostomata, and Nebaliadse) are the earliest Branchiopoda. 



morphosis" of the Coleoptera, Diptera, etc., a subsequently And we have every reason to believe that the Copepoda and 



acquired one; so also in the Crustacea those Branchiopoda Ostracoda, etc., the former with a slight, and the latter with 



with the most complete metamorphosis, such a^ Estheriadoe, scarcely any metamorphosis at all, were the earliest of all the 



Branchiopodidas, and ApusidiE, are the most recent forms, Cri;stacea ; the primeval nauplius being a low Copepod. 



