MOliPHOLOGY OF CYCLOPS. 31 



maxillipeds, and so on. It it will be seen, then, that the condition of the cord in Cyclops 

 corresponds fairly to this, only with a more complete fusion of the ganglion. In 

 Euchceta, as in Cyclops, no cells are said to be found beyond the second thoracic 

 segment *. 



The single cord found here is not only characteristic of Copepodaf, but is the 

 earliest condition in Crustacea generally. Even in Apus and Bntnchipus the resolution 

 into the ladder-arrangement is outogenetically secondary, and the want of complete 

 distinction between connective and ganglion occurs in some, at least, of the Annelida 

 (Oligochseta), and I shall revert to it again. 



Another point is the presence of ganglion-cells in the paraesophageal cords. 

 Claus, who noted the origin of the nerves of the antenna? from these cords, which he 

 calls "commissures," both in Copepoda and in the Phyllopods (including Cladocera — it 

 is quite obvious in Moina), inferred the innervation of these appendages from the sub- 

 cesophageal ganglion, a view strongly taken up by Lankester %. With the recognition 

 of the ' central ' (/. e. ganglionated) and not commissural character of these cords, the 

 superstructure founded on the misconception falls, and we are led to the admission that 

 the antennae are morphologically what they are physiologically in the Nauplius, adoral or 

 peristomial, not metastomial appendages ; and the antennules, which are always uniramous 

 in larval forms, provided, as they are, with special sense-hairs, may be again regarded as 

 belonging to the praestomium. 



From this digression we return to the nervous system and consider its histology. As 

 we found no sarcolemma, so a neurilemma seems completely absent ; the nerves are quite 

 naked. The nerve-fibres are exceedingly fine, and the only structure in the larger nerve- 

 trunks is this fibrillation, with, occasionally, a few of the fine elongated nuclei already 

 noted in the posterior part of the ventral cord. 



The inside of a bifurcation is always rounded, owing to recurrent fibrils, but contains 

 no nuclei, only dark granules. 



Ganglionic Elements. — It is most difficult to determine the relation of ganglionic cells 

 to nerve-fibres. What stain and are, in appearance, nuclei, usually seem fo lie in clear 

 spaces, representing presumably cell-cavities ; but then these nuclei, instead of being 

 spheroidal, are most irregular in shape §, though nearly uniform in size in the central 

 nervous system. Such nuclei (?) extend for some distance into the cephalic nerves. 

 Stellate cells, however, are distinctly seen in the cerebral dilatations at the base of the 

 antennulary nerves. 



A little before the termination of the sensory nerves, each primitive fibre enlarges 

 into an ovoid bipolar cell containing a large ovoid nucleus with a distinct reticulum. 

 As these bipolar cells lie close together on the adjacent fibres of the same nerve, the effect 



* Balfour, ' Embryology,' i. pp. 433, 43-1 ; Claus, iJau u. Entw. von Branchipus stagnates u. Apus caneriformis. 



t "On Apus," Quart. Journ. Microso. feci. 18.81, p. 374. 



+ In the Oorycceidce, as in Oaligus and the Parasitica generally, tho nervous centres are condensed into a thick 

 pericesophaseal ring surrounding the gullet and giving off posteriorly two cords, which supply branches to the thoracic 

 appendages and the hinder part of the body. 



S This irregularity is probably due to the post-mortem contraction of the protoplasm round the nucleus. In the 

 living animal all nuclei visible are regularly spheroidal or ovoidal. 



