MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



315 



Thus far our results tend to prove that there are in the embryo three pairs of prestomial brain 

 S'anglia: (1) the median-eye ganglia, (li) tlie lateral-eye ganglia, and (3) the central or cerebral 

 ganglia with their outgrowth, the mushroom bodies. We have been able to detect these in the 

 brain of the adult Limulus, and have been unable to And any other important fundamental 

 structures in the brain proper. 



The changes which the diflerent lobes of the brain undergo in late embryonic and in early 

 larval life have in part been described by Dr. Patten, and briefly by Kishinouye, but need further 

 elucidation. The problem is a dilticult one, and we need material for 

 it, which uo observer has yet apparently had, viz, specimens after the 

 first molt. In the trilobite stage, as seen in Kishiuouye's Fig. 45, re- 

 produced on this page, the medulla or central white fibrous portion (/) 

 is differentiated from the ganglionic cortex (</ c), but his figure does 

 not include the optic; ganglia. 



Pis. XVIII and xix represent sections of the brain of the larva. So 

 far as I have been able to make out from my sections, the cerebral i"'"- xvi.-tiio brain of LimuUis in 

 ganglia grow rapidly in size, the median-eye lobes sink in below the ^^^^ ." " " ' '' ''"' " 

 level of the upper side, the lateral-eye ganglia overlying them and form- 

 ing the two lateral superior swellings of the upper surface of the brain. The median-eye lobes 

 are partly coalesced, and the nerves proceeding from them grow together on leaving the brain, 

 only separating at the median eyes. At this period the mushroom bodies have not yet lieen 

 developed. 



XII. Observations on the development of the abdominal appendages, and of the 



GILL-SACS on BRANCHI^. PLATE XX. 



The unusual amount of interest attached to Limulus, and the great diversity of opinion which 

 has been, and still is, held by zoologists in regard to its affinities, seems mainly due to the fact 

 that this animal is a composite or generalized type, with most important features peculiar to itself 

 and to its class; it also has some points of resemblance to the Phyllopod Crustacea, which are 

 perhaps the most generalized and primitive group of Crustacea, unless the Phyllocarida be excepted. 

 On the other hand its embryology and some anatomical characters ally it still more closely with 

 the Arachuida, so much so that it seems, contrary to our earlier views, not unreasonable to sup- 

 pose that the two groups may have had a common origin; though the Merostomata, with the 

 Trilobites, were with little doubt developed much earlier than the Arachnida, the class Podosto- 

 mata being nearer their vermian ancestors than the Arachnida. 



When we take into account the Cambrian Paradoxides and the Silurian Arethusina with their 

 very numerous (about 30) segments, and what we know from Walcott's researches of the probable 

 nature of the legs of Calymene, Ceraurus, Acidaspis, etc., we can 

 see that the independent evolution of such Trilobites from some 

 branchiate Annelid was quite as natural and probable as the 

 derivation of some Protophyllo])od, or Protophyllocaridan t'rus- 

 tacean from a branchiate Annelid worm, and that it i>robably 

 took place at an earlier geological epoch. 



Examining Walcott's restoration of the underside of Calymene 

 senaria,* as well as his figures of actual sections, we notice a 

 great uniformity in all the appendages from the head to the end fig. xvii.— Restored section of the thorax of 

 of the pygidium, which is paralleled by that of the limbs of Phyl- H'^^l'^ In^'^ZTm 

 lopods. True, in the Trilobites there were no functional antennae 

 and mouth-parts, and all the limbs, both of the head, thoracic 

 region, and of the pygidium, were similar in shape, being cylindrical and short, thus fitted for 

 walking, and bore dorsal branchiie. 



The branchiai, says Walcott, as found in Calymene, Ceraurus, and Acidaspis have three forms. 

 In the first form they divide a short distance from the attachment to the basal joint of the leg and 



the epipodal oi 

 pendage. — Afte 



carapace ; 

 with the gills on 

 respiratory part of the ap- 

 Walcott. 



* Bulletin Museum Oomp. Zool., viu, PI. vi, Fig. 1, 1881. 



