292 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



of the ocellar nerve, and in nearly the same plane, arise two tegnmental nerves, and directly below 

 them a second pair of longer nerves (frontoinferior tegumental of Milne-Edwards) descend ven- 

 triilly. No nerves arise from the inferior half or two-thirds of the brain, which is smooth and 

 rounded, with no indications of a median furrow. There is also an unpaired mediofi'ontal nerve. 

 It will then be seen that, as stated by A. Milne-Edwards, there are no antenna! nerves, such 

 as usually exist in authropods with the exception of the Arachnida. This we have proved in the 

 same manner as Milne-Edwards (though at the time ignorant that he had pursued the same 

 method), by laying open with fine scissors the envelope (arterial or perinenrial) which reaches to 

 the posterior end of the brain and seeing that the fibers of the nerves sent to the first pair of 

 appendages originate quite independently of the brain itself. Moreover, after making sections of 

 several biains, it is easy to see that only the commissures connecting the braiii with the oesophag- 

 eal ring are present, the nerves to the first pair of appendages not arising from the brain itself, 

 but from the anterior and outer part of each side of the oesophageal ring, i. e., where the ring 

 joins the brain. It is to be observed that the nerves of the first pair of appendages arise in the 

 same plane as those of the second and succeeding pairs. The commissures (connectives) are very 

 short in the larva and obsolete in the adult. 



III. THE MORPHOLOGY OF THE BRAIN OF LIMULUS SEEN EXTERNALLY AS COMPARED WITH 

 THAT OF ARACHNIDA. 



The brain of Limulus is a very primitive one, more so than that of Arachnida, and should be 

 in this respect compared with that of the crustacean Branchipus. It is an arcMcerehrum,* to use 



1. opn 



iiopit 



r 



Fig. ni.— A, brain of Phalangium; B, brain of Lycosa; C, brain of Epeira i oj). n, optic nerve ; oy. J, optic lobe; c. J (and c«) cerebral lobe ; 

 md. n, mandibular nerve; r,n, rostral nerve; oe, oesophageal ring; vn (and nw), visceral nei-ve; pn (andym) pedipalpal nerve; Lopn, lower 

 optic nerve ; it. opn^ upper optic nerve. — After St. Kemy . 



the term proposed by Lankester, It sends nerves to the eyes alone and none to the appendages. 

 As we shall see in the course of our essay it is composed of optic lobes alone, together with the 

 cerebral lobes and their nucleogenous or mushroom bodies. This primitive condition places it 

 only slightly above the grade of that of Peripatus. 



It may be observed that even within the limits of the Crustacea there is a great range of 

 variation in the constitution of the supra-oesophageal ganglion. The simplest condition, that in 

 which the Crustacean brain is almost directly homologous with that of annelids, is to be found in 

 the brain of the phyllopods (Limnetidte and Brauchipodida)), where the diminutive brain is com- 

 posed of the optic and cerebral ganglia alone, the brain of these forms being the most primitive to 

 be found in arthropods. In these brains no antennal ganglia occur; they are situated behind the 

 brain. The next step is seen in Apus, whose brain, contrary to Lankester's assumption, has been 



* The terms syucerebrum .and archicerebrum appear to us to apply to very diifereut kinds of brain. The 

 arcliieerebrum of Branchipus, for example, is a very different organ from that of Limulus, both histologically and 

 morphologically. That of Branchipus is simpler and more wormlike than that of Limulus. 



