Respiration in Invertebrate Animals. 297 



General descriptive Anatomy of the Branchial Organs. 



The ultimate questions of structures will be more advan- 

 tageously studied if preceded by a few general statements as to 

 the more prominent characters of the organs dedicated to the 

 office of respiration. The Siphonostoma, Lophyropoda, and many 

 Stomapoda present no special branchial organs. 



Those of the Lamodipoda and some Stomapoda are reduced 

 to a few vesicular or cylindrical, sometimes wholly rudimentary 

 appendages which hang freely from the base of some of the feet, or 

 are inserted isolatedly at the sides of the body (PI. XVIII. fig.l). 

 The Phyllopoda are provided at the base of each of their swim- 

 ming feet with an ovoid or lanceolate branchial lamella : it is 

 distinguished from the feet by the absence of bristles. 



It is only the first and last pairs of feet in the Amphipoda 

 which are modified into respiratoiy organs. 



In the Isopoda the five pairs of post-abdominal feet are nearly 

 always concerned exclusively in the office of breathing. The 

 two multi-articulate cini of each of these feet are changed into 

 plates (PI. XVII. figs. 4 & 5), which directed backwards, are im- 

 bricatedly arranged and applied against the under surface of the 

 last caudal segment. In shape these plates differ according to 

 the species. Intermediate between the Isopoda and Decapoda, 

 the Poecilopoda in their branchial organs partake of the charac- 

 teristics of both. Inserted on the abdominal feet they resemble 

 those of the first order ; lamellar in figui'e they approach the 

 branchia of the Decapods. 



The respiratory organs of some Stomapoda (PI. XVII. fig. 3) 

 are evolved in the highest degree ; they consist of tubular tufts 

 arranged around a stalk, and float freely in the external medium. 

 In anatomical structure they fall under the type of those of the 

 Lobster (fig. 8). In Squilla these tufts exist on the ten anterior 

 feet. 



The branchiae of the Decapods are attached to the bases of the 

 anterior abdominal feet, lodged in a branchial chamber, and 

 protected by the cephalothorax. The most developed form of 

 the breathing apparatus in the Crustaceans occurs in the Deca- 

 pods. In this order not only is the function thrown upon par- 

 ticular organs entii-ely set apart for the purpose, but these organs 

 are lodged and protected within a special cavity; and the re- 

 newal of the water necessary to their operation is secured by the 

 motion of distinct a])pcndagcs or flabella. The thoracic cavity 

 is formed by a reduplication of the external tegument, and is pro- 

 vided with two orifices, one for the introduction and the other 

 for the expulsion of the fluid. Through these orifices a constantly 

 renewed supply of water is made to pass by the agency of a large 



