388 ME. H. M. BERNARD ON THE 



at this stage. In other Araclmicls, the 4th, 5th, and 6th segments have been secondarily 

 fused in various degrees of compactness. (In the Pseudoscorpions, I do not think it 

 quite certain that the 6th segment is fused ; see (oite, p. 313.) In the Spiders, the 

 fusion is remarkable in being fairly even, but the lines of constriction still appear 

 dorsally radiating from a centre. The compactness of the fusion has reached its 

 extreme in Scorpio and Phalangimn. 



In this gradual incorporation of body-segments in the area of primitive fusion, the 

 Arachnids resemble other Arthropods, but they stand quite alone in the method of the 

 initial fusion. In no other Arthropod did the fusion commence by the distortion and 

 backward dorsal prolongation of the 1st segment, for the purpose of developing the 1st 

 pair of appendages as powerful prehensile organs. In this the Arachnids are separated 

 fundamentally from all other Arthropods. 



Had this essential feature in Arachnidan morphology, revealed by the study of the Galeodidse, been 

 earlier recognized, the possibility of a close relationsliip between the Arachnids and the Merostomata 

 would not have occurred to any one. They are, in this all-important matter — viz., the primitive 

 specialization of the undifferentiated segmentation of their common Annelid ancestor — poles asunder 

 {cf. PI. XXXIV. figs. 17 A, B). Whereas the Merostomata were primitively differentiated by a back- 

 ward ventral distortion and bending of the first segment, leading to subsequent fusions of following 

 segments, the Arachnids, as we have seen, were specialized by the backward dorsal distortion and dis- 

 placement of the same segment. We fortunately have nearly all the stages of the former process pre- 

 served to us in the remains of early Trilobites and of their larvae. It reaches its extreme in Limulus 

 and the Gigantostraca, in which the ventral displacement of the mouth and distortion of the first segment 

 beyond all recognition have gone furthest. Similarly, in the Arachnids, Galeodes enables us to trace 

 the stages in the process of the specialization of the anterior segments, which reaches its extreme in 

 Scorpio and Phalancjium. Thus not only do the Arachnids and the Merostomata stand poles asunder 

 in the principle of the cai-liest specialization, but Limulus and Scorpio are in this respect extreme 

 forms, and thus still wider apart. 



A question, however, ai-ises as to whether it would be possible for an animal specialized to the one 

 extreme, say to that oi Limulus, gradually to become modified by the forward movement of the mouth 

 till it resembled the other extreme. I think it is safe to assert that this is impossible, because, in both 

 the extreme cases, the primitive segmentation is no longer traceable, and any subsequent adaptation 

 would not revive the early and vanished segmentation, but would merely change its existing specialized 

 shape. For instance, many Crustacea have secondarily re-acquired a mouth pointing anteriorly. This 

 has not taken place by any reappearance and re-tilting of primitive segments : they were lost as such 

 beyond recovery. The mouth has regained its anterior position by the modification of existing parts, 

 thereby still further enveloping the primitive segmentation in mystery. It is doubtful whether, but 

 for the Trilobites and the Phyllopods, the early method of specialization of the segments forming the 

 Crustacean head would ever have been discovei'ed. Hence, while it may not be impossible for the mouth 

 of a Limulus to return once more from its extreme posterior to the anterior position, in doing so it could 

 not once moi-e undo all the specialization of its segmentation. For in order to develop into a Galeodes, 

 which is in this respect the most primitive Arachnid, it would have to recover its long-lost Annelidan 

 segments almost in tlicir primitive condition, and then, after tilting the 1st back on to the dorsal surface, 

 further develop this primitive Arachnidan specialization till it reached the Scorpion stage. So that an 

 animal having carried one specialization to an extreme would have to undo it all, in order to try a 

 specialization the exact opposite of its own. I think it fairly safe to say that this is impossible. 



3. Terga. — The primitive form possessed a well-marked terguua on each segment, 



