1916-17.] Experiments and Observations on Crustacea. 
273 
The Pleopods. 
The pleopods have been carefully described by Pfeifer (1887), who 
pointed out that the anterior three in virtue of their rigidity probably 
function as natatory organs, and the posterior two, which are much softer, 
as simple branchise. The long axis of each of the anterior three is in its 
more proximal part concave posteriorly ; this conformation serves equally 
well for beating the water and for packing of the branchiae into small 
■compass in the fully flexed position. These pleopods can be extended 
ventrally and forwards through much more than a right angle ; the two 
posterior branchiae are hardly capable of half this amount of ventral ex- 
tension. In some of the spirit specimens the anterior three pairs had 
become fixed in extension. In both endopodite and exopodite of each 
pleopod a large blood-vessel runs along each lateral border ; the two 
vessels are connected by a great number of transverse vessels. 
The protopodite or sympodite consists of two chief segments with a 
doubtful trace of a third. The last takes the form of a single transverse 
calcification on the anterior aspect of the articular membrane joining each 
of the three anterior pleopods to the pleon. Milne Edwards and Bouvier 
(1902) have described a similar calcification at the base of the anterior 
aspect of the pleopods of Bathynomus, and Bouvier, following Hansen 
(1893) — see also Hansen (1903) — evidently regards the structure in 
question as the remains of a true segment, a view that appears to have 
met with general assent — cf. Richardson (1905, p. 133) and Caiman 
(1909, p. 204). 
The inference is by no means an inevitable one. The calcification is 
embedded rather in the articular membrane than directly united to the 
sympodite ; nor does it follow the movement of the sympodite in extension 
or flexion of the pleopod. An alternative suggestion is that the structure 
has arisen in response to functional need of some kind (cf. the develop- 
ment of muscles in relation to the intertergal articular membranes of the 
perseon), whether cle novo or by splitting of the next succeeding calcifi- 
cation. The occurrence of sesamoid bones in a mammal or bird and of 
analogous structures in the walking limbs of Glyptonot^is (see p. 257), 
interesting and fundamentally important as the phenomenon is from what 
might be called a “ physiomorphic ” point of view, is of little significance 
to higher morphology. More than this, subdivision of segments occurs 
with too great frequency in Crustacea (witness the multiarticulate sub- 
division of primitive limb segments in the “ Polycarpinea,” the double 
sternites of Glyptonotus and of Chiridotea, the split tervites of Gnathici, 
VOL. XXXVII. 18 
