806 SUMMARY OF CURRENT RESEARCHES RELATING TO 



With regard to the claims of the pair of postoesophageal ganglia 

 which lie anteriorly to the mandibular to be regarded as the first 

 ganglia of the abdominal cord, Mr. Pelseneer concludes that it is not 

 a segmental, but only an adventitious ganglion ; its lateral position 

 proves this, and it seems clear that it is enteric rather than somatic. 



To answer the question whether, to use the language of Lankester, 

 the brain of Apus is a syncerebrum or an archicerebrum, it is necessary 

 to study its histological structure ; the investigation justified the 

 former view, for a study of successive transverse sections showed that 

 the primitive cephalic ganglia end a little after the middle of the 

 brain. Towards the edge of the latter a second pair of groups of large 

 pyriform cells are to be seen ; these are the true second ganglia, 

 which belong to the first antennary pair, or first pair of the abdominal 

 cord. 



The author comes to the conclusion that both pairs of antennae 

 are metastomial ; a comparison of a number of Crustacea shows us 

 that from those with the most primitive nervous system to the highest 

 forms there are a great many intermediate stages between the condition 

 in which the nerves of the two pairs of antennse come out of the 

 cord, and that in which these nerves come out of the brain. The 

 final conclusion arrived at is that among the Crustacea there are no 

 forms with an archicerebrum ; the classification of brains of Crustacea 

 suggested by Packard is rejected. 



Embryology of Limulus polyphemus.* — Dr. A. S. Packard 

 describes the embryology of Limulus polypliemus at the stage when 

 the oval blastodermic disk, with the six pairs of the cephalic append- 

 ages, is distinctly formed ; the mouth is seen in a position in front of 

 the first pair of appendages, and from it the primitive streak passes 

 back to the posterior margin of the blastodermic disk or "ventral 

 plate."t 



The following conclusions are drawn from the observations. The 

 fact that the embryo Limulus had at first no abdominal appendages 

 (uropoda), whereas there are temporary abdominal appendages in the 

 tracheates, shows that Limulus has little in common with the Arach- 

 nida, Myriopoda, or Hexapoda. On the other hand, in the embryo 

 Crustacea the cephalic limbs are first indicated, the uropods not 

 appearing until after the Crustacea leave the egg. These facts 

 indicate that Limulus probably descended from a type in which 

 there were cephalic appendages only and no abdominal appendages. 

 The absence of a serous membrane, of an amnion, and of procephalic 

 lobes, of temporary embryonic abdominal appendages, also of proto- 

 zonites, tend to prove that the embryo of Limulus has little in common 

 with that of Tracheata. On the other hand, the earlier stages in the 

 embryology of Limulus resemble those of Crustacea in the absence of 

 the procephalic lobes ; and in the primitive development of cephalic 

 appendages alone. The comparatively early appearance of the 

 branchisB of Limulus shows that it probably never had any^ genetic 

 connection with a tracheate arthropod. 



* Amer. Nat., xix. (1885) pp. 722-7 (1 pi.). 



t See Mem. Boston Soc. Nat. Hist., 1872, figs. 12-15, pis. 3 and 4. 



