320 ME. H. M. BEKNARD ON THE 



beliincl it. Hence the formation of powerful apodemes behind it. The apodematous 

 system of the Crayfish supplies us with examples of the same rule (5). 



We may thus in Guleodcs recognize two regions of special compression, one com- 

 prising the first three segments, which resulted from the translocation of the mouth- 

 limbs ; this gave rise to the endosternite. The other comprises the three remaining 

 cephalothoracic segments ; this has given rise to the waist and diaphragm. These two 

 sejjarate phases of the specialization of the Arachnida are still clearly distinct in the 

 Galeodidse, which are almost, if not quite, alone in the retention of this primitive feature. 



In front of the endosternite of Galeodes, there is on each side a small fibrous plate 

 susjiended by means of tendon-like strands, as shown in PI. XXVIII. figs. 15 a, 16 [p). 

 Assuming these tendons to be the remains of muscular attachments, it seems not unlikely 

 that these plates are the last remains of apodemes or constrictions between the 2nd and 

 3rd segments. Sections show them to be a sponge-like arrangement of tendinous fibres 

 (fig. 19) with irregular spaces. (Compare sections on the muscular system and coxal 

 glands, where these fibrous plates will be referred to again.) 



Having already discussed the relation between the endosteruites of the Arachnids in a separate paper 

 (13), it is not necessary to do more here than to give my conclusions. 



There must originally have been a well-developed system of intersegmental membranes along the 

 whole body. In the abdominal region these have persisted to lend to that part of the body its great 

 powers of distension. In the anterior region, however, which is specialized for locomotion and 

 prehension, muscular action has drawn these membranes in to form an endoskeletal system for 

 muscular attachment. Owing to the different degrees and methods of compression of the cephalothorax 

 in the Arachnids, the endosternite necessarily varies greatly in the different families. 



In Scorpio the endosternite proper arises, as in Galeodes, from between the 3rd and 4th segments, 

 but is now complicated by secondary fusion with the diaphragm. In the Spiders, the segments of the 

 thorax being all evenly compressed, the endosternite consists of four pairs of apodemes which meet in 

 the centre, the second pair of which correspond with the endosternite of Galeodes and Scorpio, while 

 the 1st pair is perhaps represented in Galeodes by the fibrous plates above described. In Phrynus the 

 endosternite is difficult to unravel ; it may perhaps represent only the first pair of apodemes of the 

 Spiders with secondary attachment of dorso-veutral muscles. In Thelyphoniis we have a long fenestrated 

 endosternite which may correspond with that of the Spiders ; the component apodemes not, however, 

 meeting in a point. 



Other Special Apodematotis Structures. — In the constriction between segments 4 

 and 5, two pairs of chitinous processes have to be mentioned. A round hollow^ rod rises 

 from the inner end of eacli stigmatic aperture, and holds up the endosternite, so that the 

 muscles attached to its ventral and lateral surfaces should not pull it down ujoon the 

 tracbcEe (PI. XXVIII. figs, 15, 16, 18, clir). Further, two very solid folds of the lateral 

 wall rise from the anterior outer corners of the coxa3 of the 5th segment. These are 

 continuous with, and form rigid ventral attachments of, the rods (remains of the terga 

 of the 4th segment) which, on the dorsal surface, fuse with the posterior edge of the 

 tergum of the 3rd segment projecting from beneath the cephalic lobes ; these appear to 

 act like springs to support the so-called " head " {cf. p. 309, PL XXVII. figs. 1-6, and 

 PL XXVIII. fig. 15, r). 



