COMPARATIVE MOEPHOLOGT OF THE GALEODID.E. 397 



forward, the brain being forced backward. In the Spiders, in which the svxeking- 

 stomach is behind the nerve-mass, we still have traces of its former position, in the 

 persistence of a few expanding- muscle-fibres (PI. XXVIII. fig. 8). 



In Limvlus we have the arrangement which the distortion of the segments would naturally bring about ; 

 the brain lies close to the ventrally-arranged prostoraium, and the greater part of the oesophagus, specialized 

 into a kind of masticatory stomach (proventriculus), is thus morphologically behind it, actually above 

 and in front of it. The arrangement is thus the exact opposite of what we find in the primitive Arachnids. 

 If Limnlus is related as a primitive form to the Arachnids, then the position of the sucking-stomach 

 in the Spiders would have to be considered as original, and that of the other Arachnids as secondary ; 

 but, as already shown, the existing mouth-parts of Arachnids are best explained on the assumption 

 of a primitive beak, in which case the condition found in the Spiders is not primary but secondary. 



As already stated, the straining apparatus in the Arachnids is so varied that the sucking of living 

 juices must have been the primitive method of feeding. The proventriculus of Limv/us, on the 

 other band, is specialized for the trituration of solid food. Both of these appear to be independent 

 developments of some simpler Annelidan oesophagus with its expanding and contracting muscles. 



20. The Mid-gut. — The mid-gut in the primitive form stretched from the 3rd segment 

 through eleven (or tAvelve) body-segments, and was provided with at least nine pairs of 

 segmental diverticula, four in the cephalothorax, and five in the abdomen (Sco7yio). Of 

 these diverticula, those in the cephalothoracic region, i. e. in front of the waist, began to 

 degenerate as the body divided into two regions, the anterior for locomotion, the posterior 

 for digestion and reproduction. 



The mid-gut of Lhnvlus perhaps begins, as in the Arachnids, in the 3rd segment, but it is more 

 specialized. There appear to be only two diverticula which are glandular and not digestive. These two 

 probably correspond with the first two in Arachnids, i. e. if the oesophagus of Linmlus corresponds in 

 length with that of the Arachnids. If Linmlus is a primitive Arachnid, it has lost seven pairs of 

 diverticula, and those which persist are specialized into glands. It seems to me far easier to believe 

 that the specializations of Linmlus and of the Arachnids are entirely distinct modifications of some 

 undifferentiated Annelidan type than to endeavour to connect the two as nearly related. The 

 specialization of the mid-gut in Limiihis is strikingly different from that of the Arachnids. One of the 

 essential features of the Arachnidan morphology is the distensible abdomen, while the head-region is 

 specialized for locomotion and sensation. In Limulus the mid-gut and its diverticula are chiefly 

 specialized in the head-region, having lost all traces of diverticula in the abdominal region. The 

 specializations in both cases are extreme, and I repeat again that it is not likely that any extreme 

 specialization should ever swing right back to the opposite extreme, as must have been the case if the 

 Arachnids as a group are deducible from a Limulus-MVe ancestor. 



21. Heart and Circulation. — In the original form, the contractile heart ran through 

 at least ten segments (ith to 13th inclusive), being continued both anteriorly and pos- 

 teriorly by non-coutractile vessels. There were at least ten pairs of ostia drawing blood 

 from ten pairs of inter-diverticular blood-passages. As the cephalothorax became more 

 specialized, the three pairs in front of the diaphragm degenerated, except in Tkelj/phonus 

 and Galeodes. In the abdomen also the great specialization of the diverticula of the 

 mid-gut leads to variations in the number of ostia and to the general closing of the 



