CRUSTACEA OUTER ORGANISATION 



317 



like feet. They are alternately protruded and withdrawn through the shell- or 

 mantle-cleft, and serve for taking in particles of food and at the same time 

 as respiratory organs. Six pairs of tendril-like feet are found in the Lepadidce and 

 Salanidce, 3 or 4 pairs in the Abdominalia. In the Proteolepadidce and Rhizowphala 

 the tendril-like feet entirely disappear. 



6. Leptostraea. 



Nebalia, in the morphology of its trunk limbs, represents in many respects a 

 transition form between the Entomostraca and the Thoracostraca. We can distinguish 

 thoracic feet and abdominal feet (pleopoda) corresponding with the division of the 

 trunk into a thorax of 8 segments (exactly answering to the Malacostracan thorax) 

 and an abdomen (pleon) of 8 segments. The 8 pairs of thoracic feet are similar to 

 one another, as in most Entomostraca. They are 

 (Fig. 216) lamellate, strongly recalling the leaf- 

 shaped limbs of the Phyllopoda, between which and 

 j;he thoracic feet of the Malacostraca they form a 

 connecting link. Each thoracic foot of Nebalia 

 consists of the 3 parts characteristic of a biramose 

 foot, viz. a protopodite of 2 joints, an exopodite, 

 and an endopodite. The proximal or basal joint 

 of the protopodite carries a doubly-tipped branchial 

 lamella (epipodite), probably corresponding with the 

 branchial appendage of the trunk feet in the Phyllo- 

 poda. On the one side of the distal segment is 

 inserted the 5-jointed endopodite as a direct pro- 

 longation of the protopodite, and on the other the 

 unjointed exopodite in the form of a branchial 

 lamella, probably homologous with the respiratory 

 plate on the trunk feet in the Phyllopoda. FlG - 216. -Nebalia, leaf-shaped 



In the abdo m en (pleon) only the 6 anterior S eg- %* ^0^1, 

 ments carry limbs. The 6 pairs of limbs probably po dite ; ex, exopodite ; ep, epipodite. 

 correspond with the 6 pairs of pleopoda in the Mala- 

 costraca. The 4 anterior pairs (Fig. 225, D) serve for swimming ; they are typical 

 biramose feet (with proto-, exo-, and endo-podites), and show some similarity to the 

 swimming feet of the Copepoda. An epipodite is wanting. The last 2 pairs of 

 pleopoda (Fig. 225, E] are short and uniramose, and consist of one or two joints. 



c. Malaeostraea. 



The trunk feet fall into thoracic feet and abdominal feet (pleopoda) in correspond- 

 ence with the division of the trunk into a thorax of 8 segments and an abdomen 

 (pleon) of 7. We find 8 pairs of thoracic feet and 6 pairs of pleopoda, the last 

 abdominal segment being always devoid of appendages. It is better to consider the 

 extremities of the thorax and of the abdomen separately. 



The Thoracic Limbs. 



As a varying number of the anterior thoracic segments may fuse with the head, 

 a varying number of the anterior thoracic feet often enter into close relations with 

 the mouth, as accessory organs for the taking in of food (foot-jaws, maxillipedes). 

 The thoracic foot of Nebalia described above may be considered as the primitive 

 form of the thoracic feet in the Malacostraca. 



In a typical Malacostracan thoracic limb the proximal joint of the protopodite 



