RESPIRATION 141 



the sides of the funnel. Expiration results from contraction of the circular 

 muscles of the mantle, and water is driven out through the funnel. When 

 the animal is at rest these respiratory movements are gentle and rhythmi- 

 cal, and circulate water over the ctenidia. In the cuttlefish (Sepia) the total 

 branchial area is estimated at 1700-1800 cm 2 (170). 



Crustaceans and Xiphosurans. Special respiratory gills are rarely present 

 in the lower Crustacea (Entomostraca), and respiration takes place through 

 the general body surface. In the Malacostraca gills occur in connexion 

 with the appendages (modified epipodites). In lower forms they are ex- 

 posed, but in higher groups, notably the decapods, the gills are enclosed 

 in a branchial chamber. A flow of water over the branchial surfaces is 

 maintained by movements of the appendages. Stomatopods bear gills on 

 the abdominal limbs; euphausiids and some mysids have gills on the 

 thoracic legs. Among isopods the pleopods bear broad respiratory plates, 

 e.g. Ligia. 



Decapod Crustacea, which generally attain larger size than the forms 

 just described, possess well-developed gills which are lodged in a branchial 

 chamber formed by the sides of the carapace. A ventilation current is 

 drawn through the branchial chamber by baling action of the scaphog- 

 nathite (flattened blade on the maxilla), which keeps up a steady beating. 

 Water enters through inhalant apertures situated between the walking legs 

 and the lower edge of the carapace, passes across the gills and thence to 

 an exhalant aperture which lies in front of the mouth. Average areas of 

 gill surface in various crabs are: Carcinus maenas, 1-1 cm 2 g body weight; 

 Cancer pagurus, 7-8 cm 2 ; and Portunus sp., 8-7 cm 2 . Active crabs (portunids) 

 have greater gill areas than sluggish species {Libinia). In the lobster 

 (Homarus vulgaris) oxygen is mostly taken up through the gills, but the 

 swimmerets play a minor role in respiration, taking up about 3 % of total 

 oxygen respired (71, 150). 



The gills of decapods are modified in conjunction with special environ- 

 mental conditions. Gill-reduction in littoral and terrestrial crabs is dis- 

 cussed on p. 149. Alcock (1) has noted that branchial chambers and gills 

 of spider crabs and other decapods dredged from the shelf in the Indian 

 Ocean are unusually large. At these depths, around 150 m, very low oxygen 

 concentrations have been recorded in certain regions of the Indian and 

 Pacific Oceans (< 1 c.c. O a per 1.). Enlargement of branchial surface may 

 represent a functional adaptation to such conditions. 



The branchial apparatus of Xiphosurans (Limulus and allies) consists of 

 gill books, lying on the ventral side of the opisthosoma (abdomen) and 

 protected by an operculum. The gill books are built of piles of parallel 

 leaflets attached to plates or appendages of the opisthosoma. Rhythmic 

 movements of the gills cause water to circulate over the gill lamellae. 

 Transection experiments point to the existence of respiratory centres in 

 abdominal and suboesophageal ganglia (155). 



Fishes. The branchial apparatus of fishes takes the form of gill filaments 

 which are borne on gill arches lying between the pharynx and branchial 



