THE NAUPLIUS STAGE. 



191 



md^ 



occur here. The U-shaped antennal gland (at) opens externally on 

 the basal joint of the second antenna. The oral aperture which 

 lies between the two antennae is overhung by a large upper lip 

 (lab rum), and leads into the alimentary canal, in which can be 

 distinguished a short oesophagus, a widened mid-gut, and a hind- 

 gut. The anal aperture may be wanting in the first Nauplius 

 stages (Cttochilus, Cyclops). It has been observed in many cases 

 that this aperture origin- 

 ally opens on the dorsal 

 surface (Cirripedia, Fig. 

 103, p. 212; Cetochilus, 

 Fig. Ill B, p. 233; em- 

 bryonic stage in the 

 Cladocera, Fig. 72, p. 

 147), the aperture shift- 

 ing later to the posterior 

 end of the body between 

 the furcal processes. The 

 nervous system retains 

 its original connection 

 with the ectoderm, and 

 consists of the supraoeso- 

 phageal ganglion, circum- 

 oesophageal commissures, 

 and the first pair of gan- 

 glia of the ventral cord. 

 The second antennae are 

 innervated from a pair of 

 ganglia lying behind the 

 mouth (Claus, Dohrn), 

 an arrangement which is retained by the Phyllopoda in the adult. 

 As a sensory organ, there is the unpaired Nauplius eye, which lies 

 in the frontal region, and is composed of three parts. The most 

 developed muscles are those of the limbs, which are attached to 

 a point in the mid-dorsal region. The heart is not yet developed. 



Although there is no external segmentation in the Nauplius body, we can 

 nevertheless recognise in it the following segments:— An anterior pre-oral or 

 primary cephalic segment, a posterior terminal or anal segment at the posterior 

 end of the body, and the true trunk-segments which He between these. As 

 members of the last group, we recognise the section of the body belonging to 

 the second antenna and the mandibular segment. We have, however, already 

 pointed out (p. 165) that the region of the first antenna possibly also corresponds 



Fig. 89. — Nauplius of Cyclops (after Claus). a>, first 

 antenna ; o", second antenna ; md, mandible ; o, 

 Nauplius eye ; at, antennal gland ; ds, mid-gut with 

 excretory cells. 



