INTERNAL ANATOMY OF PHYLLOPODS. 403 



it, and thus the brain of the shrimp or crab represeni s the brain of the 

 ThylloYiod ])lus the oesophageal 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. 

 Clans'* 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. 



lu the Calanidoj there is a distinct brain from which arises the first 

 antennal nerves, while in the Corycieidce the ventral cord is fused with 

 the brain. 



It will thus be seen that the Phyllopods possess the simplest, most 

 primitive form of bria-in, characterised by the lack of antennal nerves. 

 If we were to confine ourselves simply to the Apodidce and BrancM- 

 podidae, 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 

 iu 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 Ajjodidce is called by Lankester, in his paper on Apus, 

 an arcMcerehrum, while the composite brain of "all Crustacea, excepting 

 Apus, and possibly some other Phyllopods," he denominates a syncere- 

 hrtim. As to the nature of the brain of Limulus, Professor Lankester 

 states that " the only other case amongst adult Arthropods, in whicli 

 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 oesophageal ring of Limulus, 

 Avhich is fundamentally made up of ganglia with cross-commissures, it 

 appears to us that we have a parallel to the ladder-like arrangement 

 of the postoral head-ganglia of the Phyllopods. 



The histological structure of the archi-cerebrum of Limulus is more 

 complicated than in that of the Phyllopods, which, so far as we have been 

 able to see, is slightly more complicated than the brain of the Chieto- 

 pods, judging by Leydig's excellent figures (Taf. IV). 



The following provisional grouping of Crustacean brains appears to 

 be justified by known facts, although, except the brain of Decapoda and 

 liimulus, no special histological work has been accomplished: 

 { Decapoda. 

 I Tetradecapoda. 



Entomostraca. 

 [^ Cirripedia. 



*Ziir Kenntuiss der Organization iind des feinern Banes der Daphniden nndver- 

 wandter Cladocercn. Vou C. Clans. Zeit. wissen. Zool. XXVII, 187(5, 362, Tat XXVI, 

 figs. 8-10. 



