HABIT AND HABITAT IN THE GALATHEIDEA. 87 



the thoracic nerve-ganglia observed iu any of the Galatheidea, the con- 

 dition almost approaching that which characterizes the Brachyura 

 (see Plate 1, Figs. A, B, C, D, E). The last thoracic leg (appendage XIII), 

 which in the Galatheidea is modified into a slender cleaning-organ, of 

 course does not participate in the clinging action or in locomotion, and 

 it is noticeable that, even in Porcellana platycheles, the ganglion of the 

 13th segment remains semi- isolated from the central mass formed by 

 the fusion of most of the other thoracic ganglia. 



The marked clinging-habit of Porcellana plafycheles should be con- 

 nected not only with the resistance to dislodging forces, but also with 

 the general protective " melting into the background." The body is 

 flattened, and when at rest is pressed closely against the rock — the 

 chelae are flattened and expanded in the horizontal plane, and are notched 

 so as to fit accurately against the front edge of the carapace — and the 

 whole dorsal body surface is coated with shaggy, grey, insensitive hairs, 

 nearly matching in colour the rock to which the animal clings (see Plate 

 2, D). In this connection we remark also the absence of those trans- 

 verse ridges of the carapace which are so conspicuous in more active 

 members of the group. These carapace ridges in G-alathea are fringed 

 with hairs of a fairly simple, once-pinnate type (Plate 2, A, and Plate 

 4, B), whose continual agitation by the motion of the surrounding water 

 probably prevents the settlement of such unwelcome guests as Hydrozoa, 

 Polyzoa, etc., likely to retard the swimmer by increasing the friction- 

 surface. A sedentary form like Porcellana platycheles has no such in- 

 terest in avoiding encrustation — and in it we find no trace of ridges or 

 of waving hairs, the shaggy hairs of the dorsal surface being too close 

 and coarse to be easily agitated by movement of the water. Again, the 

 ridges, if present, would inevitably detract from the general protective 

 resemblance to surroundings, and from this standpoint it seems likely 

 that the presence of encrusting organisms may be of positive advantage 

 to the crab. At all events, Spirorbis is quite commonly found attached 

 especially to the chelae of P. platycheles — a position likely to ensure 

 commensalism (Plate 2, D). 



II. Modifications which tend to prevent choking by mud. 

 A Macrurous form such as we suppose the Galatheid ancestor to have 

 been, swimming in the clearer open water, required, and probably pos- 

 sessed, no special devices for protection of the branchial cavity from 

 stoppage : we find in the Lobster, Penaeus, etc., a notable absence of 

 such protective devices. Here, the gills are just sufficiently well guarded 



