30 T. H. MORGAN. 



the stomodseum invaginates quite early and perhaps simul- 

 taneously with the early formation of mesoblast at this place. 

 Further, Schimkewitsch has called attention to the fact that the 

 stomodseum of Spiders in its earliest development is a triangular 

 invagination, and he compares it directly with the triangular 

 invagination of the oesophagus of the Pycnogonids. 



3. The early formation of body-cavity surrounded by meso- 

 blast in the legs of Spiders has an exact parallel in Pallene and 

 Phoxichilidium. Yet, however tempting such a comparison 

 may be in this connection, it must be admitted I have not con- 

 clusively proved this to be true for the Pycnogonids, but only 

 exceedingly probable. 



4. In both Arachnids and Pycnogonids we have well marked 

 diverticula from the mid-gut into the legs. In the Pycnogonids 

 these go into the chelicerse and the four pairs of walking legs, 

 and the same holds for the Spiders ; but from a comparison of 

 the appendages of the two groups we must suppose that the 

 second pair of Pycnogonid's appendages to have lost their diverti- 

 cula, and the last appendages either to have acquired diverticula, 

 or more probably inherited them together with the appendage. 

 In Chelifer the diverticula appear very early in development and 

 contain some of the yolk from the mid-gut. This is shown very 

 distinctly in Metchnikoff's figures for Chelifer, and in this respect 

 the embryo resembles closely the embryos of Pycnogonids. 



5. In both Arachnids and Pycnogonids the first pair of ap- 

 pendages are chelate. This in itself would draw attention to 

 the similarities of the two groups, but we know further that in 

 both groups this first pair of chelate appendages is innervated 

 from the brain. These facts were considered by Balfour suffi- 

 ciently important to indicate alone a relationship between the 

 groups. He says: " The presence of chelate appendages inner- 

 vated in the adult by the supra oesophageal ganglia rather points 

 to a common Phylum for the Pycnogonida and Arachnida, 

 though, as shown above, all the appendages in the embryos of 

 true Arachnida are innervated by post-oral ganglia." 



I have not been able to find any post-oral ganglia for Pallene, 

 but the first pair of appendages arises on the sides of the stomo- 

 dseum and later moves forward. In this respect it compares 

 closely with the Spiders, and the early innervation of this pair 



