HABIT AND HABITAT IN THE GALATHEIDEA. 



91 



from below, which might meet and check the outgoing current (see Plate 

 4 and Text Figure 1). 



Various members of the group show an interesting series of devices 

 ensuring steadiness of the breathing current — such devices being par- 

 ticularly necessary in animals with the enclosed type of branchial cavity. 

 Prof. Pearson, in his Cancer Memoir (L.M.B.C), has pointed out the 

 existence in that part of the carapace which floors the branchial cavity 

 of a ridge, which by its presence helps to maintain a steady current of 

 water past the gills. Specializations of an apparently somewhat similar 

 function are visible in certain of the Galatheidea. The branchial cavity 

 of Galathea and of Munida is long and narrow, with a slight spiral twist, 



Fig. 1. — Diagram-Longitudinal section through the front end of the body of one of 

 the Galatheidea. 

 v.b. = ventral body-wall. 

 ap. =branchiostegite of one side, 

 br.ca. abranchial cavity of that side. 

 ex. = direction of ex-current stream. 

 p. =line of a theoretical in-current. 



m. = position of matted hairs on the ma.\illipede bases, which prevent the 

 entrance of the current p. 



and is floored (unlike those of Brachyura) by the body-wall itself : we 

 trace in this floor the line of a deflnite, though not prominent, longitudinal 

 ridge external to a groove along which the main body of the breathing 

 current appears to flow (Plate 3, A, i and ii). 



Porcellana has a corresponding specialization more nearly of the 

 'Cancer type : the branchial cavity is partially floored (i.e. towards its 

 front end) by a special portion of the edge of the branchiostegite, which 

 is peculiarly twisted to produce a ridge doubtless similar in function to 

 the rather more definite ridge of Cancer (Plate 3, B, and Plate 1, G). 



Porcellana shows a remarkable respiratory habit — that of temporary 

 suspension of the breathing activities on alternate sides of the body. 



