328 



COMPARATIVE ANATOMY 



CHAP. 



Decapoda are always originally epipodial appendages on the basal 

 joints of the protopodites of the thoracic feet. They may be considered 

 as homologous formations, and perhaps correspond with the gills of the 

 Phyllopoda and the basal branchial plates (epipodites) of Nebalia. 



The pouch-shaped Amphipodan gills have already been considered. 



Schizopoda. The branchial tufts of the Euphausidcc (Fig. 226), whose branches 

 are feathered, project freely from the basal joints of the protopodites of the thoracic 

 limbs into the surrounding water without being covered by the lateral lamellse of the 

 cephalo-thoracic shield. The branching of the gills becomes increasingly complicated 

 from the anterior to the posterior thoracic limbs. On the most anterior thoracic 

 limb the gill is a simple appendage. 



In Lopliogaster we find 2 to 7 such gills. These consist of 3 feathered 

 branches, the upper one of which lies in a branchial cavity covered by the lateral 

 lamella of the cephalo-thoracic shield. The gills are said not to rise direct from the 

 basal joint of the protopodite, but close to it from the body. None the less they 

 should be considered as dislocated epipodial appendages. 



The gills of the Decapoda (Figs. 227, 228) deserve more detailed description. 

 Over the sides of the thorax to the right and left there is always an arched extension 

 of the cephalo-thoracic shield, which, as branchiostegite (Fig. 227, led), covers a 

 respiratory cavity (kh) in which the gills () lie. The branchiostegite extends 

 ventrally to the points of insertion of the thoracic limbs, where the respiratory cavity 

 on each side communicates by means of a longitudinal slit with the surrounding 

 medium. We distinguish in the first place, according to their manner of insertion, 

 three sorts of gills podobranchise, arthrobranchiae, and pleurobranchise. The podo- 

 branchise arise from the basal joints of the thoracic limbs, the arthrobranchise from 

 the articular membranes between the basal joints and the body, and the, pleuro- 

 branchise from the body itself, but directly above the basal joints of the thoracic 

 limbs to which they belong. All gills are to be considered phylo-genetically as 

 epipodial appendages of the basal joints of the protopodites, the arthro- and pleuro- 

 branchife having moved from their original places. Again, we distinguish two sorts 

 of gills, according to their special form, viz. trichobranchue and phyllobranchise. 

 In the trichobranchise numerous branchial filaments stand round a common stem 



Id, 



FIG. 229. Birgus latro. Diagrammatic transverse section in the region of the heart (after 

 Semper), kd, Branchial or lung cover ; h, heart ; 1:, gills ; ah, respiratory cavity ; p, pericardium ; 

 ek, branchial blood-canals leading to the heart ; a\, 02, 03, 4, lung or shell vessels leading from the 

 heart ; Ib, respiratory tufts ; el, pulmonary vessels leading to the heart ; eli, the same near their 

 entrance into the pericardium. 



or a common axis, like the bristles of a bottle brush. In the phyllobranchiae the 

 branchial filaments are small lamellfe arranged in two rows on the stem, like the 



