32 T. H. MORGAN. 



wards by the abdomen, but at the same time the sixth pair of 

 ganglia also is carried dorsalwards and above the fifth pair, from 

 which the connecting nerve fibers pass upwards to the sixth. At 

 the present time it is impossible to determine whether this is due 

 to a mechanical adjustment between the shifted abdomen and the 

 last pair of thoracic ganglia, or whether the sixth pair, belonging 

 properly to the abdomen, has taken part in the general shifting 

 of that structure. 1 



There are certain objections against this comparison which I 

 have attempted, and if they do not directly oppose yet do not sup- 

 port the hypothesis. First and most important is the uncertainty 

 of brain invaginations in Pycnogonids. These seem to be present 

 in all the Arachnids, and easily seen in the development of the 

 embryo. In the Pycnogonids I have not been able to find such 

 invaginations. We have seen in Fig. 19 that the groove might 

 possibly bear such an interpretation ; but even if this were true, 

 we would expect to find a much more pronounced involution, but 

 this does not seem to be the case. Again, the ventral organs, 

 which havebeen compared with those of Peripatus,lend nosupport 

 to the hypothesis. It is possible that the Pycnogonids have come 

 from the Arachnids, at a time when these latter have had such 

 organs in common with the ancestors of the Insects, and that 

 they have been fully retained in the Pycnogonids. Lastly, the 

 openings of the reproductive organs. Typically the ovaries and 

 testes of the Sea-Spiders are a pair of organs united posteriorly by 

 a cross commissure. They extend into the walking legs and 

 open on the second joint of these. There are many exceptions 

 to this, but they are regarded as secondary. The openings on 

 the legs have no homologue in the Arachnids, except perhaps in 

 Limulus, nor does it furnish any ground for comparison with 

 other Arthropods. But if we assume the group to have come 

 directly from the Annelids, we have no better ground here for a 

 comparison. We are greatly in need of observations on the 

 development of the sexual openings, and until we get such, the 



1 If we assume with Hatchek a common descent for all Arthropods, and that 

 in the Insects we have several of the anterior segments about the mouth, sup- 

 pressed, we might assume that the Spiders have lost a third pair of appendages, 

 and the Pycnogonids retained it, and in this way bring into line the other 

 appendages of the groups. 



