THE AMERICAN LOBSTER. 



193 



The respiratory organs.— In the adult lobster there are twenty pairs of gills, oue 

 of which, belonging to the second pair of maxillipeds, is rudimentary. There are 6 

 podobranchise, 10 arthrobranchiae, and 1 pleurobranchiae, distributed according to the 

 followiug table: 



Table 37. — Branchial formula. 



Thoracic segments and appendages. 



Podo- 

 branclriae. 



Artbrobrancliiaj. 



Plouro- 

 branckiae. 



Totals. 



Anterior. 



Posterior. 



VII, first maxilliped 



0(ep.). 



liud.(ep.). 

 1 (ep.). 

 1 (ep.). 

 1 (ep.). 

 l(ep.). 

 1 (ep.). 

 





 

 1 

 1 

 1 

 1 

 1 

 





 



1 

 1 

 1 

 1 

 1 









 

 

 



1 

 1 

 1 

 1 



(ep.). 



1 rud. (ep.). 

 3(ep.). 



3 (ep.). 

 4(ep.). 

 4(ep.). 

 4(ep.). 

 1 



VIII, second maxilliped 



IX, third maxilliped 













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 podobranchise of the following segments are 

 very small and are partially exposed, together with their reniform epipodites. In the 

 second larva the podobranchia are covered by the carapace (plate 21) and the branchial 

 formula is complete (fig. 101, plate 31). 



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 31) the podobranchia carries four rows of 

 filaments, and the mastigobranchia, or epipodite proper, is a long, tapering, hairy 

 plate. 



THE VISUAX, 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 anteunules. 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 (149), 



F. C. B. 1895—13 



