2S PEOF. M. M. HAETOG ON THE 



It seems probable, from a comparison of the oldest Nauplius with the youngest 

 Cyclops-stage, that tbe gland is the same structure in both, but that its opening lias 

 acquired different relations in the metamorphosis. In the Phyllopod Nauplius, Claus 

 describes an " antennary gland" in the same position as in the Copepod Nauplius, and 

 opening below the masticatory hook. This he identifies with the " antennary gland " 

 of the Malacostraca, and describes as disappearing soon. In the meantime appears the 

 proper shell-gland of the adult, having its aperture, as in Cyclops, at the outer segment of 

 the second maxilla. Claus lays great stress on the presence of two pairs of excretory 

 tubes as indicative of Anuelidan affinities. But in the absence of any fuller account of 

 the development there remains the possibility that here, as well as in Cyclops, the 

 two glands are really one in origin, and that the connection with the antenna is only 

 replaced by that with the second maxilla. In this case it is interesting to note that 

 the primitive relation of the duct to the antenna is retained in the Malacostraca, in 

 which the" shell-gland " proper, with its duct opening in the second maxilla, is apparently 

 never developed *. 



This is interesting, as showing the archaic relation, seen only in the larva of the 

 ancestral group of Entomostraca, preserved and functional in the more recent Mala- 

 costraca. 



The Nervous System and Sense-Organs. 



Central part. — This is composed, as in all Arthropods, of a prooesophageal ganglion 

 or brain, two paraesophageal cords, one on each side of the gullet, and a ventral 

 cord, which extends here to the anterior boundary of the sixth thoracic segment, where 

 it bifurcates. 



Brain (PL IV. tigs. 3, 4). — This is an irregular mass, squarish in both sagittal and 

 transverse section ; but while in the latter its edges are respectively superior, inferior, and 

 lateral, in the sagittal plan the square is, as it were, tilted, so that its somewhat hollow 

 postero-inferior face rests on the lower half of the upper limb of the gullet, and its anterior 

 angle abuts against the median eye. The cellular elements form everywhere a thick super- 

 ficial layer, in front occupying half the thickness of the brain, and showing here a distinctly 

 paired grouping. They are scanty in front, next the eye ; and they are absent from the 

 postero-inferior face and the part of. the brain in front of this, being the great transverse 

 commissure, and showing consequently a fine molecular structure in sagittal section, 

 sharply marked off from the other nucleated part. In dissections and horizontal sections 

 the arrangement of these fibres can be seen. The hinder set are transverse and go from 

 one parcesophageal commissure to the other ; the next are oblique, and connect the 

 one side of the brain with the parcesophageal commissure of the other side ; the 

 anterior set are transverse, and connect the two sides of the brain f . The brain gives off 



* Except that a looped gland with openings at mx" occurs in Elaphocoris and Acanthosoma larva) of Sergestids 

 (Claus, Crust, p. 36, t. iv., 1, fig. 12, p. 37). 



■f These are well seen in some dissections I have made of Calanus, and coincide on the whole with Claus's figure of 

 the t>rain of Calanelkt (Freil. Cop. t. vii. p. 9), except that he does not figure or notice in the text the posterior 

 transverse commissure. The grouping of the nuclear elements into three paired sets, as seen in Calanus and 

 Calanella (Clans), cannot be made out in the brain of Cyclops, so much more consolidated. 



