TRANSACTIONS OF SECTION D. 609 



small and thin, is an efficient cleaner of the limb-bases and of the branchial 

 cavity. 



Clinging has led to natural selection of a relatively short body, while breadth 

 has increased. This has gone so far in Porcellana that the grappling legs are there 

 the radii of a circle rather than of an ellipse. In P. platycheles, more especially, 

 the legs are almost regularly disposed around the circle, and each has the shape 

 of the arc of a circle, securing firmness of attachment without excessive rigidity. 

 Broadening of the body has given increased breadth to the branchial cavity, and with 

 broadening has gone a flattening down of the carapace, reducing the danger from 

 side blows. The result of this is that the long axes of the gills are sloping in 

 Galathea and nearly horizontal in Porcellana; the chinks between their component 

 leaflets are therefore almost vertical, and thus less likely to harbour foreign 

 particles than the horizontal chinks between the gill leaflets of a Penseus. The 

 branchiostegite, with its inbent edge, protects the branchial chamber of Galathea 

 better than the same structure does that of Penaeus. This chamber is. still better 

 protected by carapace and leg-bases in Porcellana, and, with the Galatheid cleaning 

 limb in addition, there cannot be such need of epipodites, at any rate for protection 

 and cleaning, as in the Fen(eid<e. In addition to the plate of the first maxillipeds, 

 we find epipodites in both Galathea and Porcellana on the third maxillipeds, the 

 chelipeds and the first two walking legs ; they are, however, reduced, and more so, 

 especially the posterior ones, in Porcellana than in Galathea. 



The chelipeds of Galathea and P. longicornis transfer food to the mouth, and 

 in the latter also assist locomotion, being pressed against a surface irregularity 

 and then partly straightened, with the result that the body is pushed backwards. 

 P. platycheles is too completely flattened for the chelipeds to bend easily in towards 

 the mouth region ; but if food is once secured in the cavity between and beneath 

 them, it is likely to be swept into the mouth by the maxillipeds, while the thick 

 row of long hairs stretching down from the outer edges of the chelipeds must 

 help to stop all but very small particles. In Macrura and Galathea the respiratory 

 stream enters between the leg-bases, especially the posterior ones ; but in Porcellana, 

 P. platycheles more especially, the entrance channel is almost restricted to the gap 

 just behind the chelipeds. Here the general hairiness of the species is specially 

 marked, and one or other of the walking legs is brought close up against the 

 cheliped, so the channel is narrow and barred by an efficient straining apparatus. 

 The highly developed protective arrangements for the branchial cavity allow P. 

 platycheles to thrive where neither Galathea nor P. longicornis could survive, and 

 partly account for the greater tenacity of life of the first named. 



The antennules are important water-testers, and are periodically cleaned by 

 the hair-comb on the inner side of the third maxillipeds. The flicking of the 

 antennules and sweeping of the maxillipeds seem to create a current forwards, 

 which might bring grit against the sensory hairs. The teeth and notches of the 

 rostrum of Galathea localise that current in the forward notch on either side, and 

 this notch diflers from the others in possessing a thick row of hairs for straining. 

 P. lonfficornis has the rostrum reduced, but a row of upright hairs on either side, 

 a little way behind the front edge of the carapace, perform the straining function. 

 These hairs are not as conspicuous in P. platycheles, but the general hairiness 

 must here hinder the passage of minute fragments over the carapace-edge. The 

 third maxillipeds, further, brush the front of the carapace. 



Every moderately active clinging animal may fall, and must have a method of 

 righting itself. Galathea needs this less, for, even when reversed, it can progress 

 by flapping the abdomen till it comes beneath a stone to which it can cling. 

 Sometimes, however, it grips usually with the third pair of walking legs on either 

 side, and by contracting their muscles and flapping the abdomen at the same time it 

 is able to turn over, using the back edge of the body as an axis. 



In Porcellana, straightening of the third walking legs raises the body, and 

 the flapping of the abdomen is then just sufficient to jerk the back of the carapace 

 backwards till the vertical position is reached and passed and the animal has 

 turned over. The great claws are meanwhile brought backwards to diminish the 

 moment opposing the righting movement, P. platycheles is more light-shy than 



1906. B R 



