THE DECAPODA 275 



joint, and very rarely as in Laomedia (Thalassinidea) the endopodite 

 is similarly divided. 



Among the Anomura the uropods are variously modified. In 

 the Galatheidea they retain more or less the type of structure 

 which they showed among the Macrura. In most Paguridea they 

 become modified as organs for fixing the posterior end of the body 

 in the shell or other lodging carried by the animal, the rami are 

 stout and curved, with roughened, " file-like " surfaces which are 

 l)ressed against the shell, and the appendages of the two sides share 

 in the asymmetry of the Avhole abdomen. In the Lithodidae alone 

 among Anomura the uropods are wanting. This is all but 

 universally the case also among the Brachyura, where only in 

 certain Dromiacea (Dromiidea) are there found traces of uropods in 

 the form of small plates intercalated on each side between the last 

 abdominal somite and the telson. 



Branchial System. — With the single exception of the aberrant 

 genus Leucifer, all Decapoda possess branchiae connected with some 

 or all of the thoracic somites and lying in the cavities enclosed by 

 the branchiostegites on each side. The typical number of branchiae 

 which may be present on each side of a somite is four, arranged as 

 follows : One is attached to the lateral wall of the somite dorsal 

 to the articulation of the appendage {pleurohranchia), two to the 

 articular membrane between the coxopodite of the appendage and the 

 body-wall (arlhrohranchiae), and one, representing a differentiation 

 of part of the epipodite, is inserted on the coxopodite itself 

 (jwdobranchia). 



Four series of gills corresponding to these can be traced in a 

 more or less incomplete form throughout the whole series of the 

 Decapods. They are, howevei', not invariably distinguished from 

 each other by the position of attachment in the manner just 

 described. In particular, the distinction between arthrobranchiae 

 and pleurobranchiae is often very difficult to draw in practice, 

 and there are some cases where an arthrobranchia in one species 

 is plainly homologous with a pleurobranchia in another. Glaus 

 has shown that in the development of Penaeus three bud -like 

 outgrowths appear on the proximal part of most of the thoracic 

 limbs (Fig. 164, A). The distal one (a) gives rise to the epipodite 

 with its podobranchia and the two othei's (b, c) are the arthro- 

 branchiae. As development proceeds an apparent change in the 

 position of these last is l^rought about by coalescence of the proximal 

 part of the appendage with the body, so that the branchiae no longer 

 a[)pear as outgrowths of the limb but spi-ing from that part of the 

 body-wall which afterwards foi'ms the articular membrane of the 

 joint. The pleurobranchia appears a little later than the other two 

 (Fig. 164, B, d), but its place of origin is very close to if not actually 

 on the basal part of the limb itself. Williamson has observed a 



