30 PEOF. M. M. HAETOG OX THE 



cord ; the mandibles and paragnatha? receive nerves that run outwards and forwards, and 

 the nerves to the next appendages are almost transverse ; hut by the time we get to the 

 third segment they are markedly oblique, running backwards to the appendage of the next 

 segment. The parietal cutaneous nerves appear everywhere to come off, as in the two 

 last thoracic segments, from the segment anterior to that which gives nerves to the 

 limb. This arrangement is, perhaps, explicable on the hypothpsis that the segments were 

 originally mesoblastic, and that each intersegmental septum received both an anterior and 

 posterior nerve, which passed from it to the body-wall, and that with the tendency to 

 centralization of the body the anterior nerve alone persisted. 



To this account of the nerves it is necessary to add that in each segment a pair of 

 nerves are given off to the trunk-muscles, and that there appears to be a fine muscular 

 nerve, distinct from the large sensory, to some at least of the appendages. The sensory 

 nerves appear to spring from the ventral face of the cord, the muscular from a higher 

 level. The ventral nerve-cord is perforated in the maxillary region by two holes * for 

 the retractor muscles of the postoral bar. Most of the fibres constituting the great bulk 

 of the ventral cord are longitudinal ; but both vertical and transverse commissural fibres 

 occur, the latter at short intervals. The cord is traversed by distinct longitudinal 

 lacunae of rounded section, which in transverse section are seen to be laterally sym- 

 metrical, and can be traced in sagittal section for a considerable distance ; these would 

 doubtless favour the osmotic changes required for the active life of the cord. 



The branches of the bifurcation of the ventral cord run at first below and then along- 

 side the intestine, giving off branches. In the third abdominal segment each splits into 

 a dorsal branch for the fourth segment and the anus &c, and a ventral one running to 

 the furca, both branches presenting those ganglionic enlargements, to be referred to 

 hereafter, characteristic in Invertebrata of sensory nerves near their termination, as 

 shown by Leydig. 



We are now in a position to note critically the morphological peculiarities of this 

 nervous system. Paired ganglia never occur in the ventral cord of the Copepoda ; the 

 ganglia and their connectives are strictly median, when distinguishable, as in the 

 Calaniclce. In these the ventral cord forms, according to Glaus f, a ganglionated chain 

 (though in Diaptomus and Calanus nerve-cells exist also in the constricted or com- 

 missural intervals). This is the typical arrangement, according to Claus : — two nearly 

 fused ganglia in the region of the oral appendages, a third behind the maxillipeds, a 

 fourth and fifth behind the first and second thoracic appendages respectively, and a 

 sixth and seventh close together at the hinder part of the third or beginning of the 

 fourth segment. The nerve to the first oar-foot comes from the ganglion behind the 



* The perforations of the central nervous system hy muscles connected with the gullet is very puzzling, and I am 

 at a loss to give any morphological explanation of this singular state of affairs, repeated in Calanus and Diaptomus, 

 and prohahly common to the order. Can the short commissure immediately behind the gullet in Phyllopods represent 

 the part of the metoesophageal ganglion in front of these perforations ? A puzzling monstrosity occurred in one live 

 specimen of Cyclops I examined ; the ganglion of the fourth thoracic segment was pierced completely from above 

 downwards by a large oval hole, occupied by nothing, but a mere solution of continuity. 



+ Freil. Cop. p. 41. 



