26 THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 



The mesodermic somites are hollow, the somatic portion lining the cavities of 

 the appendages, the splanchnic portion not entering the appendages. At this stage 

 there is posterior to the last thoracic appendage a swelling marked OP in figure 

 LXXV1II. Although this was not observed in all cases, it is apparently a normal 

 structure, for it is present in later stages. It corresponds in position to the 

 operculum of Limulus. Figure LXXIX is a drawing of a longitudinal section of 

 a still more advanced embryo. The first two abdominal appendages ( A & B in Fig.) 

 are not well marked but the mesoblastic somites corresponding to them are well de- 

 fined (A' & B' in Fig.) Figure LXXIX represents a more highly magnified portion 

 of the same section. 



The two abdominal appendages, A & B, are better defined in reality than in the 

 figure. 



It will be seen that the ectoderm covering the two appendages, A & B, is 

 columnar, each cell having a well defined chitinous boundary and a square nucleus. 



These cells correspond, in histological structure, to those forming the laminae of 

 the lung book of the adult spider as described by McLeod.' 1 ' The histological 

 structure of the cells of the gill lamellae of Limulus is quite similar. 



On the anterior surface of the appendage, A, will be seen a fold (I in figure.) 

 If we imagine the appendage to be pushed further in and come to lie entirely in the 

 lung cavity L, B, the fold on the anterior face of the appendage and others which 

 may arise there will then lie on the anterior wall of the lung cavity, but these folds 

 will then be directed backwards and not forwards as before the involution of the 

 appendage and will consequently correspond in every way to the laminse of the lung 

 book. All the stages in the involution of the appendage were not traced, but there 

 can be little doubt that the lung book of the spider results from the involution of 

 embryonic abdominal appendages. Such an involution of appendages to form the 

 lung book has been suggested by Lankester ( ' 2) on theoretical grounds, but not from 

 observation. The involuted appendages are covered by the structure described as 

 the operculum. (OP, Fig. LXXIX'.) 



Some remarks on the supra-oesophageal ganglion and structures connected with 

 it will conclude the observations made on the embryology of spiders. 



(1) Archiv. de Biologic, 1884. 



(2) Quarterly Journal 1885. 



