398 MR. H. M. BEENAED ON THE 



inter-divei'ticular blood-passages, traces of which, however, persist in the veno-pericardial 

 strands. 



The heart of Limulus is not unlike that of the primitive Arachnid. It appears, however, to have 

 commenced in the 3rd segment, and to have extended hack to the 10th (incl.). The veno-pericardial 

 connections perhaps arose in the same way as in the Arachnids, and would therefore indicate the former 

 presence of more alimentary diverticula, which have since degenerated. These resemblances between the 

 two animals are perfectly explicable as their common inheritance from their Annelidan ancestors. It is 

 obvious that the hearts are not strictly homologous, inasmuch as that of the primitive Arachnid 

 extended through more segments than does that in Limulus. 



22. Besjiiration. — The primitive form had respiratory invaginations commencing in 

 the 4th segment of the cephalothorax, and extending at least as far back as to the 8th 

 segment of the abdomen. The series was clearly in close physiological connection with 

 the ostia of the heart and the inter-diverticular blood-passages {cf. PI. XXXIII. fig. 12), 

 A respiratory invagination projected into each of the last-named. A primitive arrange- 

 ment like this will explain all the present arrangements found in modern Arachnids, 

 both alimentary, circulatory, and respiratory. The form of the respiratory invagination 

 was probably some sunple (or j)erhaps slightly branched) chitin-lined tube from which, 

 in the manner described above, either lung-books, tuft-trachese, or branching tracliese 

 could be deduced in adaptation to the subsequent specialization of the alimentary or 

 circulatory systems. 



One of the main arguments for attempting to connect Limulus with the Arachnids is the apparent 

 possibility of deducing the luug-books from the gills of Limulus. But the backing-up of hypothesis by 

 hypothesis can only give the appearance of strength. The only attempt at evidence in favour of this 

 demonstration is that deduced from embryology, in which it is shown that the gills and the lung-books 

 make their first appearance exactly like one another. It is not easy to see what else could be expected. 

 Given the respiratory invagination opening on the posterior face of rudimentary limbs, a sagittal section 

 passing through such an invagination would naturally give the appearance of a limb with the lung-book 

 on its posterior face like the gills of Limulus. The embryological evidence decides nothing, and 

 the ultimate appeal has to be made to comparative anatomy. 



The answer to be given, for instance, to the question " Were there ever flat lamellate limbs like those 

 of Limulus on the abdomen of the primitive Arachnid ? " goes far to settle this matter. AVhat actual 

 evidence we have as to the character of the abdominal limbs shows that they were filamentous jointed 

 appendages like those on the cephalothorax, but hardly so highly specialized. Indeed, neither in shape 

 nor in number do the remains of limbs in the Arachnids correspond with those oi Limulus. 



Again, the sensory plates on the three joints of the pectines of Scorpio, which are supposed to be 

 persistent remains of the gills of a Limuloid ancestor, ai'e seen in sagittal sections to be on the ventral 

 face of the limb, and not on the posterior or upper face. They are closely parallelled by the racquets on 

 the same three joints on the last leg of Galeodes, which certainly could not have been dedviced from 

 branchial leaflets turned into sensory processes, if such a transformation is possible. The branchial 

 leaflets were not on the first three joints, but on a specialized area of the Limulus leg comparable with 

 the gill of Apus and the epipodite of the Crustacea. 



Further, the invagination of four pairs of gills on segments iii.-vi. of the abdomen will not account for 

 the position of the tracheie in any other Arachnid except Scorpio. It will not account for the tracheje 

 in the cephalothorax in Galeodes, nor for the two kinds of tracheae, tuft and branched, in the abdomen. 



