The points of agreement in the number and character of the appendages, form of the abdomen, etc. between the various types of Zoæa appear to me too striking to be explained in the manner attempted by Claus. It seems improbable that a peculiarity of form acquired by the larva of some ancestral Malacostracan should have been retained so permanently in so many groups[207]—more permanently indeed than undoubtedly ancestral forms like that of Mysis—and it would be still more remarkable that a Zoæa form should have been two or more times independently developed.
There are perhaps not sufficient materials to reconstruct the characters of the Zoæa ancestor, but it probably was provided with the anterior appendages up to the second pair of maxillipeds, and (?) with abdominal swimming feet. The heart may very likely have been many-chambered. Whether gill pouches were present on the maxillipeds and abdominal feet does not appear to me capable of being decided. The carapace and general shape were probably the same as in existing Zoæas. It must be left an open question whether the six hindermost thoracic appendages were absent or only very much reduced in size.
On the whole then it may be regarded as probable that the Malacostraca are descended from Protophyllopod forms, in which, on the adoption of swimming habits, six appendages of the middle region of the body were reduced or aborted, and a Zoæa form acquired, and that subsequently the lost appendages were redeveloped in the descendants of these forms, and have finally become the most typical appendages of the group.
The relationship of the various Malacostracan groups is too difficult a subject to be discussed here, but it seems to me most likely that in addition to the groups with a Zoæa stage the Edriophthalmata and Cumaceæ are also post-zoæal forms which have lost the Zoæa stage. Nebalia is however very probably to be regarded as a præ-zoæal form which has survived to the present day; and one might easily fancy that its eight thin thoracic segments with their small Phyllopod-like feet might become nearly aborted.
Copepoda. The Copepoda certainly appear to have diverged very early from the main stem, as is shewn by their simple biramous feet and the retention of the median eye as the sole organ of vision. It may be argued that they have lost the eye by retrogressive changes, and in favour of this view cases of the Pontellidæ and of Argulus may be cited. It is however more than doubtful whether the lateral eyes of the Pontellidæ are related to the compound Phyllopod eye, and the affinities of Argulus are still uncertain. It would moreover be a great paradox if in a large group of Crustacea the lateral eyes had been retained in a parasitic form only (Argulus), but lost in all the free forms.
Cirripedia. The Cirripedia are believed by Claus to belong to the same phylum as the Copepoda. This view does not appear to be completely borne out by their larval history. The Nauplius differs very markedly from that of the Copepoda, and this is still more true of the Cypris stage. The Copepod-like appendages of this stage are chiefly relied upon to support the above view, but this form of appendages was probably very primitive and general, and the number (without taking into consideration the doubtful case of Cryptophialus) does not correspond to that in Copepoda. On the other hand the paired eyes and the bivalve shell form great difficulties in the way of Claus’ view. It is clear that the Cypris stage represents more or less closely an ancestral form of the Cirripedia, and that both the large bivalve shell and the compound eyes were ancestral characters. These characters would seem incompatible with Copepod affinities, but point to the independent derivation of the Cirripedia from some early bivalve Phyllopod form.
Fig. 237. Figures illustrating the development of Astacus.
(From Parker; after Reichenbach.)
A. Section through part of the ovum during segmentation. n. nuclei; w.y. white yolk; y.p. yolk pyramids; c. central yolk mass.
B and C. Longitudinal sections during the gastrula stage. a. archenteron; b. blastopore; ms. mesoblast; ec. epiblast; en. hypoblast distinguished from epiblast by shading.
D. Highly magnified view of the anterior lip of blastopore to shew the origin of the primary mesoblast from the wall of the archenteron. p.ms. primary mesoblast; ec. epiblast; en. hypoblast.
E. Two hypoblast cells to shew the amœba-like absorption of yolk spheres. y. yolk; n. nucleus; p. pseudopodial process.
F. Hypoblast cells giving rise endogenously to the secondary mesoblast (s.ms.,). n. nuclei.