COMPAEATIVE MOEPHOLOGY OF THE GALEODID^. 315 



The origin of the waist or diaphragm in the Arachnida is not far to seek. It divides the body into 

 the two well-defined regions above described, the anterior knit together by muscles to form a specialized 

 locomotory mechanism, the posterior a distensible bag for the digestive and the reproductive apparatus. 

 It seems to me that the development of the former region would necessarily draw in the intersegmental 

 constriction between the two regions. 



The function of the diaphragm will be better explained in the section on the alimentary system. It 

 supplies, as we shall see, a kind of neck to the digestive bag, which serves to protect the organs of the 

 cephalothorax from undue pressure on the part of the contents of the abdomen when it is greatly 

 distended. 



The Segments — Special Structures. 



Having thus reviewed the general segmentation of the body, we have to describe 

 certain special structures connected with the segments, such as the beak, the eyes,, 

 and the limbs. 



First Segment: the Beak. — The beak* is a marked feature in the Galeodidse. It 

 projects anteriorly from between the basal joints of the pedipalps, ventrally to the 

 chelicerai. The mouth being at its tip, we may safely assume that it is composed of a 

 labrum and of a ventral labium fused together along their edges. It is represented in 

 PI. XXVII. figs. 11, 14, and in sections in PL XXVIII. figs. 3, 4, 5, 6. 



The labrum, which is in all Arachnids a structure sui generis, is keeled dorsally and 

 very firm ; the keel may originally have been due to the jn'essure of the chelicerse on 

 each side. It is, moreover, quite immovably fixed to the coxal joints of the pedipalps 

 (PI. XXVIII. fig. 3), W'hich are themselves rigidly attached to the ventral skeleton. 

 Anteriorly, an arrangement of feathered setse projects from the labrum in such a way as 

 to form a sieve for the straining of the animal- juices, which form the characteristic food 

 of the Arachnids. The setae are arranged in a vertical row on each side of the mouth, 

 the two rows lying very close to each other (PI. XXVII. fig. li). The setae of the same 

 side are joined together by cross-pieces, so that the sieve has regular rectangular meshes 

 (PI. XXVIII. fig. 6). 



The labium, w'hich is applied all along to the base of the labrum, so as, with it, to 

 form a beak, ends anteriorly in two fleshy lobes (PI. XXVII. fig. 14), each provided 

 with a long, feathered, tactile hair. The tip of the labium seems to be movable by 

 means of muscles, but the exact mechanism is difficult to make out. In the ventral 

 middle line, the labium is strengthened by a chitinous plate folded along the median 

 line. This is a prolongation of the sternal plate betw^een the coxae of the pedipalps ; 

 the whole plate may be considered to be the fused sternites of the 1st, 2nd, and 

 3rd segments {cf. PI. XXVII. fig. 14 with PI. XXVIII. fig. 3, and p. 311). 



I have not hesitated to call the parts composing the beak the labrum and labium, 

 because I can see no reason why they should not be so. They form the upper and lower 

 maro-ins of an anterior mouth ; the labrum has the typical position of a labrum or 

 prostomium (i. e. if we take into account the shifting forward of the chelicerse), and the 

 labium is the protruded anterior margin to the sternal surface bounding the mouth 

 posteriorly. As ah'cady suggested, this protrusion is no doubt due to the distortion of 



* " Eostre buccule,"' " labium haustelhforme,'' " languette sternale," &c. of authors. 



42* 



