THE CRUSTACEA 



9 



Fir,. 4. 



Leaf-like trunk-limb of Lepidurus (Bran- 

 chiopoda). (After Sars.) en, endites ; ex, 



exites. 



more complex than the simple biramous type. Lankester has 

 called attention to the lobed leaf-like appendages of the Branchiopoda 

 (Fig. 4), as probably approximating to the ancestral form. As will 

 be shown below, it is not altogether 

 easy to recognise the homologies 

 of the various lobes even within 

 the limits of the group Branchio- 

 poda, and their exact relation to 

 the parts of the biramous limb is 

 still open to doubt, but it is 

 probable that the Branchiopod limb 

 preserves characters belonging to 

 an early phyletic stage before the 

 biramous type had become fixed. 

 It does not seem profitable to 

 go beyond this and to attempt, 

 as some have done, to compare 

 the limbs of the Branchiopoda in- detail with the Polychaete 

 parapodium. 



The general character of the modifications which the original 

 type of limb undergoes is often, though by no means always, 

 plainly correlated with the functions which the limbs discharge. 

 In swimming-limbs the rami are often flattened and oar-like, and 

 fringed with plumose hairs or flattened spines. For walking or 

 creeping one of the rami, generally the inner, is stout and cylindrical, 

 tipped with a claw, and having the segments connected by definite 

 hinge-joints allowing movement only in one plane. The oral 

 appendages have the gnathobasic lobes developed at the expense of 

 the rest of the limb, the rami persisting, if at all, only as sensory 

 " palps." A multiarticulate flagelliform modification of the rami is 

 generally associated with a sensory (tactile or olfactory) function, 

 as in the antennules and antennae. A pincer-like (chelate or sub- 

 chelate) form is frequently assumed by limbs used for prehension, 

 the terminal segment being flexed against the penultimate, or 

 opposed to a thumb-like process of the latter. 



Special Morphology of Limbs Ocular Peduncles. In many 

 Crustacea, notably in the Anostracous Branchiopoda and in the 

 majority of the Malacostraca, the eyes are set upon peduncles 

 which are movably articulated with the head, and which may be 

 divided into two or three segments. The view that these peduncles 

 are homologous with the limbs was first suggested by H. Milne- 

 Edwards, and has been widely but not iiniversally accepted. In 

 spite of much discussion, however, it cannot be said that the point 

 has been finally decided. The fact that the eye-stalks are most 

 fully developed and most distinctly articulated not in the more 

 primitive forms, but among the highly specialised Decapoda, is 



