NOTES ON THE EMBRYOLOGY OP LIMULTTS. 
547 
of the coalesced terga of the second antennal and the mandi- 
bular segments, and the “ cervical suture ” indicates merely 
the line of their junction. In the crab the homologous suture 
is to be sought on the deflexed surface of the carapax below 
the lateral margin. The mandibular terga are comparatively 
small, and are the epimera of H. Milne Edwards . 1 It is need- 
less to say that I do not accept the opinions of Young (’80) on 
this subject, nor his views of the supra-oesophageal ganglia. 
This same view is perfectly applicable to the Arachnida, where 
otherwise we should have to regard the eyes as distributed on 
different segments in different species and even in the same 
species. A further confirmation is found in the fact that in 
the Scorpions (nor so far as I am aware in any Arachnid) the 
cephalothoracic tergum does not segment, or show any signs of 
the metamerism so evident on the ventral surface. This point 
seems to me an additional argument in favor of the union of 
Limulus with the Scorpions. 
The post-oral nature of the first pair of appendages and the 
non-appendicular nature of the metastoma needed to render 
valid the comparisons of Professor Lankester have been 
previously suspected or proved, and my observations are only 
confirmative, though in the case of the metastoma conclusive 
proof was hitherto lacking. 
Unfortunately Metschnikoff’s account of the development 
of the mesoblast is very scanty owing to the fact that he did 
not employ sections, and so I can only refer to Balfour’s 
account of the Arachnida. The process in Limulus and 
Agalena is closely similar and in its details considerably 
different from that found in Crustacea. In the latter group 
the mesoblast does not as a rule become divided into somites , 2 
nor does the schizoccele extend into the legs at first. In both 
Agalena and Limulus it arises as a single broad sheet, which 
later divides into two bands which migrate to the region of the 
appendages. Then the coelom is formed by splitting and 
1 ‘ Hist. Nat. Crust.,’ pi. i, fig. 9, b. 
2 It. does so in Mysis (Metschnikoff, teste Balfour) aud in Cyclops (Urba- 
nowicz in ’84). 
