THE RELATIONSHIPS OF THE SEA-SPIDERS. 



151 



BMBBYO PALLENE 



>-/- 



d-" 



Fig. 3 



legs. The second and third pairs of appendages are 

 absent from the embryo at this stage. If we had cross- 

 sections through the body of 

 such an embryo, we should 

 find, lying at the base of each 

 of the walking-legs, a small 

 cavity with a definite boundary 

 of mesoderm cells ; this cavity 

 is the body-cavity. Such sec- 

 tions would further show us 

 that the digestive tract had 

 begun already to push out into 

 the appendages, and cacJi pouch 

 contains yolk, as does also the 

 whole of the mid-gut. 



It is not necessary to follow 

 the later stages of development of such an embryo, as 

 we have already seen those peculiarities which are to 

 be used in the comparison of the Pycnogonids with 

 other groups. 



Now in the briefest manner possible I wish to point 

 out the importance of these embryological facts, and 

 their bearing upon the relationship of the Pycnogonids 

 to the group of the Arachnids. 



First there is the multipolar delamination, which is 

 such a striking feature in the development of the 

 Pycnogonids ; so that when we come across an exactly 

 similar series of phenomena in the false-scorpions 

 (Metschnikoff) and in the spiders (Balfour), we have 

 found a strong point for comparison in the develop- 

 ment of the two groups. We find in the spiders the 

 first external appearance of the embryo to be a round 



