90 K. ZIMMERMANN. 



clifiging habit and the radial working of the legs, which entail move- 

 ments likely to endanger organs occupying the position of epipodites. 

 We should probably connect with this habit of spreading the legs radially 

 (a habit which prevails throughout the Galatheidea, though it is most 

 marked in Porcellana) the peculiarities of the general gill-formula of the 

 group, which is characterized by the absence of podobranchs and im- 

 portance of the pleurobranchs. Some of the leg movements involved 

 would seem to be a source of danger to gills in the position of podo- 

 branchs (near the outer edge of the branchial chamber), exposing them 

 to a risk of friction against one another and against the edge of the 

 branchiostegite. Reduction of the organs exposed to this danger is 

 accompanied by increased importance of the gills of the two inner series 

 (arthrobranchs and pleurobranchs). (See Plate 1, F and G.) 



In connection with the ascent of the shore by Porcellanidse, the altera- 

 tion in shape of the branchial cavity is noticeable, the shape of the cavity 

 being practically of a type intermediate between those found in the 

 swimming Galathea and the established (unrelated) shore-form, Cancer. 



Porcellana flatycheles, a sluggish species inhabiting the muddiest 

 part of the mud-zone, shows a marked degree of specialization of the hairs 

 which guard the inhalent aperture. These hairs, as well as those of the 

 coxopoditic tufts, are of a much-branched and twice-pinnate type, and 

 exactly resemble those which, in members of the Brachyura, constitute 

 so effective a strainer, being totally dissimilar to the corresponding 

 structures in Galathea (any species). (Plate 4, G.) 



It is remarkable that P. longicornis, unlike flatycheles, has none of this 

 complicated structure : all the hairs near openings of the branchial 

 cavity are alike of the simple type characteristic of Galathea squamifera 

 (Plate 4, B). 



It is necessary to bear in mind the difference in habitat of the two 

 species, P. platycheles being a typical mud-dweller, w^hilst longicornis 

 frequents such shores as those of Guernsey, where igneous rocks weather 

 into reefs separated by steep gullies, swept perfectly clear of mud by the 

 tidal currents. 



With specializations which tend to promote efficiency of the breathing 

 current we must class a peculiar growth of hairs found on the basal joint 

 of the third maxillipede of all species so far examined. These hairs are 

 short and strong, furnished with short, jagged saw-teeth, and are matted 

 together to form a kind of felt-work across the gap between the maxilli- 

 pedes of the two sides. This acts as a forward extension of the floor of 

 the branchial cavity, and seems to guard against the entrance of water 



