BBANCHEF OF AETHEOPODA. 



241 



arrangements by which the exchange of gases between the blood 

 circulating within the appendages, and the surrounding medium 

 is more easily effected, and causes either the whole appendage 

 or a fork of it to become a respiratory organ. 



A farther differentiation on the same lines, leads to an increase 

 in the number of the respiratory lamella; of an appendage, or to 

 the formation of filamentous structures; the reason for all these 

 changes is a necessity for increasing the surface. These organs are 

 branchiaa. The connection between the branchias and the appen- 

 dages as seen in the Vermes is clearly a foreshadowing of the still 

 more developed arrangement in the Crustacea, which has here 

 become typical. But it is, of course, more than doubtful whether it 

 has been directly derived from the Vermes. 



The gradual development of gills may be followed out step by 

 step through the Crustacea; the functions of respiration and of 

 locomotion are often so closely united that it is difficult to say 

 whether certain forms of these appendages should be regarded as 

 gills, or as feet, or as both com- 

 bined. The conversion of locomotor 

 into respiratory organs can not un- 

 frequently be made out in the suc- 

 cessive appendages of one and the 

 same individual. The branchiferous 

 metameres vary greatly, so that we 

 may say that the appendages of 

 each segment can form gills, or 

 supports for them by developing 

 branchial organs from one of their 

 two primitive branches. The num- 

 ber and special structure, as well as 

 the position, of these respiratory 



organs varies. 



Where the feet themselves be- 

 come gills, they have the form of 

 broad thin lamellaa (Fig. 124, A br), 

 the surfaces of which are adapted 

 to effect the exchange between the 

 blood that is circulating in them 

 and the surrounding water. Organs 

 of this kind are common among the 

 Branchiopoda, in which, as a rule, 

 a large number of feet become gills, 

 while, in addition to these, peculiar 

 pouch-like appendages may be seen 

 to be specially entrusted with the 

 respiratory function. The abdominal feet of the Isopoda form 

 branchial lamellaa. In the Amphipoda the gills are tubular 

 appendages of the thoracic segments, which are ordinarily attached 

 to the basal-joints of the feet. In the Stomapoda the original form 



R 



Fig. 121. Sections of Crustacea. 

 .A of a Phyjlopod (Limuetis) 

 (after Grube). B of Squilla (after 

 Milne-Edwards), c Heart, i Enterou. 

 n Ventral medulla. br BranohiaB. 

 d Fold of the dorsal integument, 

 whicb forms a shell in A. 



