318 



COMPARATIVE ANATOMY 



CHAP. 



carries an epipodite, while tlie exopodite and the 5-jointed endopodite are attached 

 to the distal joint. Very often both the exopodite and the epipodite disappear, and 

 the thoracic foot is then an unbranched, 7-jointed limb. The proximal joint of the 

 protopodite occasionally fusing with the skeleton of the thorax, the distal joint alone 

 is recognisable. 



Arthrostraca (Fig. 217). — Here, where the most anterior thoracic segment is 

 fused with the head, the most anterior pair of thoracic feet are associated with the 



Fio. 217.— Thoracic limbs of some Artlirostraca. A, 1st pair of thoracic feet (maxillipedes) of 

 AmpMtlioe penioillata (Costa). B-B, Apseudes. B, 1st right thoracic foot ; C, 3d thoracic foot ; 

 B, 2d thoracic foot (after Boas). E and J?, Asellus. £, Sd thoracic foot ; J?, 1st thoracic foot (after 

 Boas). I, 17, Joints of the protopodite ; 1-5 joints of the endopodite ; ?:, fcj, masticatory ridges ; ep, 

 epipodite ; ex, exopodite ; en, endopodite. 



oral limbs as a pair of maxillipedes. The absence of the exopodite is the general 

 rule for the thoracic feet of the Arthrostraca. The proximal joint of the protopodite 

 often fuses with the thoracic skeleton. On the basal joint of some thoracic feet 

 there is, in the female, a lamellate appendage, the brood plate or lamella. The brood 



plates cover over a cavity, the brood pouch, 

 on the ventral side of the thorax, and into 

 this pouch the eggs enter and there develop 

 (Fig. 218). 



The first thoracic foot (maxillipede) is 

 characterised by the fact that the distal joint 

 of the protopodite, and in the AmiMpoda the 

 j)roximal joint of the endopodite as well, car- 

 ries a masticatory plate (lacinia). 



The AmpMpoda are distinguished by the 

 fact that the middle and posterior thoracic 

 feet carry on the basal joints of their proto- 

 podites pouch-like gills (epipodites) (Fig. 218), 

 Fio. 21S.— CoropMum longlcorne {Am- which, however, by the fusing of the basal 

 jiUpod). Transverse section through the joints with the skeleton of the trunk may be 

 thorax (after Delage.) cJ, Intestine ; 7,, ij^^erted directly on the latter. They are 

 heart; &»i, ventral chord ; Z, liver ; &r, gdl ; 4. j- . j ^^^^J ^^^ 



brl, brood lamella ; ov, eggs in the brood !^°* aii^^cted outwards m the way character- 

 cavity ; 6/, thoracic feet. istio of epipodial appendages, but they rise 



from the inner side of the basal joints. The 

 gills of the Caprellidm (Fig. 198, p. 295) are generally limited to the 4th and 6th 

 pairs of thoracic feet, and these thoracic feet are then reduced to the proximal joint 

 of the protopodite. In the Isopoda gills are wanting on the thoracic feet, except on 



