Introduction to Animal Morphology. 337 



pair have fine slits in them, through which air enters into 

 caecal tubes. In Stomapods the gills are fringes at the bases 

 of the five abdominal feet. They are branched processes of 

 the limbs of the cephalothorax, either floating freely (Thysano- 

 poda), or roofed in by a process of the carapace according as 

 they arise from the bases of the thoracic feet or from the body 

 wall ; in these cases, as swimming does not provide for a change 

 in the water bathing the gills, there are thin, flat, external pro- 

 cesses from the bases of the maxillipedes which create in- 

 ternal currents. In Amphipoda the 5-6 pouch-like gills are 

 often covered by elongations of integument, sometimes closing 

 in a perfect (Typhus), or imperfect gill cavity (Sergestes). 

 In Poecilopoda the first pair of abdominal feet, by the basal 

 union of their proximal joints, form an operculum, under 

 which are the gills. Caprella has two short gills. In Lopho- 

 gaster there are not only dorsal, but ventral gill filaments, 

 projecting freely between the feet. Euphausia has the gills 

 of the first pair of limbs as a single pouch-like process. In 

 Decapods the gills are lamellar, twenty in Nephrops and 

 Homarus, twenty-one in Scyllarus. The outer wall of the 

 gill chamber has in some crabs a number of glandular swell- 

 ings, which in Gecarcinus and other land crabs are large 

 and spongy, and keep the gills wet. When the gills exceed 

 the feet in number, the supernumerary lamellae are attached to 

 the first and second pairs of limbs. 



The nervous system has long commissures uniting 

 the pre-oesophageal with the post-oesophageal gan- 

 glion, and a ventral chain of ganglia, primarily paired, 

 but usually united medially. The nerves often arise 

 along with, and apparently from, the commissures. 



The brain ganglia are small and separate, with a median 

 commissure in the parasitic Copepods. In Lepadidae the 

 ventral chain consists of 4-5 ganglia, while in Balanidae there 

 is but one, compound, condensed. In Poecilopoda there is a 

 large pharyngeal mass, united by three commissures under the 

 pharynx, and giving a double branch to the trigonal tail. In 

 manyAmphipods the size of the brain depends on that of the 



Z 



