242 COMPARATIVE ANATOMY. 



gives rise to another arrangement — the five pairs of abdominal 

 swimming-feet bear at their base a mesially-directed tuft of branched 

 branchial filaments (B br). 



From the Schizopoda to the Decapoda we meet with a con- 

 tinuous series leading from the simplest relations to the most com- 

 plicated ones. Distinct gills are not unfrequently wanting in the 

 former (Mysidas), or the gills have the form of ramified additions 

 to the appendages of the cephalo-thorax floating freely outwards 

 (Thysanopoda). A fold is gradually developed, from the dermal 

 skeleton of the cephalo-thorax which forms a plate covering in a 

 lateral space above the thoracic feet (p. 235). In this space the gills 

 are placed; it becomes a laterally-placed closed branchial cavity (Deca- 

 poda), which is in connection with the surrounding medium by means 

 of a cleft left between the free edge of this lamella, and the basal 

 portion of the feet. As the covering lamella of the branchial cavity 

 becomes more closely approximated to the ventral surface of the 

 body, the primitively simple longitudinal cleft, which gave entrance 

 to the water, becomes divided into two portions, and so gives rise to 

 a larger posterior, and a smaller more anteriorly placed opening, by 

 which the water which has entered by the larger opening, passes 

 back to the exterior, after it has bathed the gills. The gills may 

 separate themselves somewhat from the base of the feet, and arise 

 from the wall of the branchial cavity, but even then they often 

 correspond in number to the appendages. In most of the Decapoda, 

 however, the number of gills is greatly increased, the most anterior 

 of the ambulatory feet being provided with several gills, while some 

 of the maxillipedes also share in this arrangement. The respiratory 

 appendages are more distinctly differentiated in the Pcecilopoda, 

 where the anterior appendages have no appended organs, while the 

 five pairs of feet attached to the abdomen are converted into broad 

 plates, and carry a large number of branchial lamella3. 



§ 187. 



A more rapid exchange of water around the branchial organs is 

 effected in various ways. Most simply, when the appendages them- 

 selves function as gills, or when the gills, notwithstanding their 

 being special organs, are attached to the swimming-feet. The action 

 of the appendages produces a continual change of water around 

 the organs, and puts respiration and locomotion into direct relation 

 with one another. The appendages of the Branchiopoda and the 

 swimming-feet of the Stomapoda may be cited as examples of this 

 arrangement. In others the exchange of water is effected by a special 

 covering-plate of the branchia) which is formed from the modified 

 abdominal feet ; this is the case in the Pcecilopoda and the Isopoda. 

 The water can be renovated, even when the animal is at rest, by the 

 continual movement of this covering-plate. 



The formation of a branchial cavity leads to the differentiation of 

 new arrangements, by which the exchange of water is effected. In 



