INTERNAL ANATOMY OF PHYLLOYODS. 403 
it, and thus the brain of the shrimp or crab represents the brain of the 
Phyllopod plus the cesophageal ring of the latter. We have seen that 
in the larval Branchipus the two pairs of antennal nerves actually do 
arise from masses of ganglion cells. These two masses may form the 
two pairs of antennal lobes in the Decapodous brain, which is therefore 
probably an aggregate of three pairs of ganglia. 
_ The brain of the Phyllopods is more primitive than in the Cladocera. 
Claus* figures of the brain of Daphnia magna show that the first an- 
tennal nerves arise from the brain, while the second antennal nerves 
arise some distance back from the succeeding pair of ganglia. 
Iu the Calanida there is a distinct brain from which arises the first 
antennal nerves, while in the Coryceide the ventral cord is fused with 
the brain. 
It will thus be seen that the Phyllopods possess the simplest, most 
primitive form of brain, characterized by the lack of antennal nerves. 
If we were to confine ourselves simply to the Apodide and Branchi- 
podide, in which the body is much elongated, we should attribute the 
want of concentration of the brain peculiar to the Phyllopods as due to 
the elongation of the body and to the exceptional number of arthromeres 
composing the body, but we see the same structure and form of the brain 
in Limnetis, the most generalized form in the suborder, where the body 
in lack of differentiation approaches the Cladocera. Hence the ner- 
vous system of the Phyllopods does not seem to have been borrowed 
from the Crustacea standing below them. 
The brain of the Apodide is called by Lankester, in his paper on Apus, 
an archicerebrum, while the composite brain of “all Crustacea, excepting 
Apus, and possibly some other Phyllopods,” he denominates a syncere- 
brum. As to the nature of the brain of Limulus, Professor Lankester 
states that “the only other case amongst adult Arthropods, in which 
it appears with certainty that the so-called cerebral ganglion is a pure 
archi-cerebrum, is that of Limulus,” although he adds (p. 375), ‘I should 
wish, however, to guard against the inference that I consider any close 
affinity to obtain between Apus and Limulus.” 
We are disposed to agree with the view that the brain of Limulus is 
a genuine archi-cerebrum, comparable with that of the Phyllopods, and 
regard this as corroborative proof that Limulus is a Crustacean rather 
than an Arachnidan, no true archi-cerebrum being known to exist in 
adult Arachnida. Furthermore, in the csophageal ring of Limulus, 
which is fundamentally made up of ganglia with cross-commissures, it 
appears to us that we have a parallel to the ladder-like arrangement 
ot the postoral head-ganglia of the Phyllopods. 
The histological structure of the archi-cerebrum of Limulus is more 
complicated than in that of the Phyllopoeds, which, so far as we have been 
able to see, is slightly more complicated than the brain of the Cheto- 
pods, judging by Leydig’s excellent figures (Taf. LV). 
The following provisional grouping of Crustacean brains appears to 
be justified by known facts, although, except the brain of Decapoda and 
Limulus, no special histological work has been accomplished: 
( Decapoda. 
| Tetradecapoda. 
Phyllocarida. 
Cladocera. 
Entomostraca. 
Cirripedia. 
Syncerebrum J 
*Zur Kenntniss der Organization und des feinern Baues der Daphniden und ver- 
wandter Cladoceren. Vou C, Claus. Zeit. wissen. Zool. XX VII, 1576, 362, Taf. XX VI, 
figs. 8-10. 
