THE AMERICAN LOBSTER. 
193 
The respiratory organs . — In the adult lobster there are twenty pairs of gills, one 
of which, belonging to the second pair of maxillipeds, is rudimentary. There are 6 
podobranclihe, 10 arthrobranchiae, and 4 pleurobranclihe, distributed according to the 
following table: 
Table 37 . — Branchial formula . 
Thoracic segments and appendages. 
Podo- 
branchiae. 
Artlir obranchiae . 
Pleuro- 
brancliiae. 
Totals. 
Anterior. 
Posterior. 
VII, first maxilliped 
0 (ep.). 
0 
0 
0 
0 (ep.). 
VI LI, second maxilliped 
1 rud. (ep.). 
0 
0 
0 
1 rud. (ep.). 
IX, third maxilliped 
1 (ep.). 
1 
1 
0 
3 (ep.). 
X, first pereiopod 
1 (ep.). 
1 
i 
0 
3 (ep.). 
XI. second pereiopod 
1 (ep.). 
1 
1 
L 
4 (ep.). 
XII, third pereiopod 
1 (ep.) • 
1 
1 
1 
4 (ep.). 
XIII, fourth pereiopod 
1 (ep.). 
1 
1 
i 
4 (ep.). 
XIV, fifth pereiopod 
0 
0 
0 
i 
1 
Total 
6 (ep.). 
5 
5 
4 
20(1 rud.). 
ep. = epipodite. rud. = rudimentary. 
The first larva has no rudiment of a podobranchia in the eighth somite, but all 
the other branchiae are represented. The podobrancliiee of the following segments are 
very small and are partially exposed, together with their reniform epipodites. In the 
second larva the podobranchige are covered by the carapace (plate 21) and the branchial 
formula is complete (fig. 101, plate 34). 
The gills are developed in the embryo as simple folds or pouches in the body 
wall. 1 They belong to the trichobranchiate type, the respiratory surface being gradu- 
ally increased by growth of the multiserial branchial filaments. 
In the fourth larva (fig. 106, plate 34) the podobranchia carries four rows of 
filaments, and the mastigobrau cilia, or epipodite proper, is a long, tapering, hairy 
plate. 
THE VISUAL ORGANS AND APPENDAGES. 
The ocellus . — The median eye, which is present in the first larva, is situated at 
the apex of a prominent median papilla, between the paired eyes and antennules. It 
is marked by a pear-shaped mass of dark pigment. It disappears in the course of 
larval life, and no trace of it can be seen in the adult. The ocellus was observed by 
Sars {175) in the first larva of Homarus gammarus. 
The paired eyes . — The compound or lateral eyes originate in the embryo as disk- 
shaped thickenings of ectoderm, and do not become lobate until a relatively late period 
(cuts. 27-34). 2 In the summer eggs eye-pigment is developed when the embryo is 
about 27 days old. It then appears as a thin line or crescent-shaped area, when seen 
from the surface. The eye-spot increases gradually in size, and its characteristic 
shape affords a convenient gauge to measure the embryonic development. (Plate J.) 
In the first larva the eye is relatively very large. It is dorso-ventrally compressed 
or flattened, as in the embryo and in all subsequent stages. The stalks are propor- 
tionally shorter than in the fourth larva, and since they nearly meet in the middle 
line in front of the brain, they are practically sessile and immobile. 
1 For an account of the development of the Decapod gill see 94, p. 392, figs. 193, 230-233. 
2 The structure and development of the compound eyes of the lobster have been carefully worked 
out by Parker (449). 
r. C. B. 1895—13 
