THORACICA. 211 



■dorsal shield and terminates in a caudal fork. The anal aperture 

 •(o/) lies somewhat far forward on the dorsal side of this part of 

 the body. Between it and the posterior edge of the dorsal shield 

 there arises from the dorsal side of the thoraco-abdominal section 

 a large spine running backwards and ending in a sharp point (the 

 so-called caudal spine, ds). 



In later stages, on the ventral side of thoraco-abdominal region, in 

 front of the caudal fork, are found a pair of thorn-like processes 

 repeating the shape of the larger caudal fork, and in front of these 

 another smaller pair. These thorn-like processes Avhich, in some 

 forms, are still more developed, may produce the appearance of 

 segmentation on the thoraco-abdominal section, but cannot be 

 regarded as indicative of a true segmentation of that region of the 

 body. 



In the anterior region of the body, the unpaired Xauplius eye 

 which is closely applied to the brain is always distinctly perceptible. 

 The oral aperture is overhung by a large helmet-shaped upper lip 

 (ol), which, by its form, recalls the upper lip of the Phyllopodan 

 larva. The limbs show the structure typical of the NaujpUus stage, 

 and, in the first stages, often appear only indistinctly jointed, while 

 in tlie later stages the joints are more numerous and more distinct. 

 The most anterior limb (a', first antenna) is invariably uniramose, 

 and its distal portion is beset with swimming setae. The second and 

 third limbs (a", mcl) are biramose. The longer exopodite consists of 

 a large number of closely crowded joints beset with swimming setae ; 

 the shorter endopodite has fewer joints. The protopodite in both 

 these limbs carries several masticatory hooks directed towards the 

 ■oral aperture. 



The later stages are characterised by the gradual increase in size of 

 the body, by the appearance of the above-mentioned spinous processes 

 on the dorsal shield and in the thoraco-abdominal region, and above 

 all by the presence of a paired frontal sensory organ {t's), which here 

 ■develops in the form of two filamentous processes inserted near the 

 Nauplius eye. Among the internal organs, besides those already 

 mentioned (eyes, brain, glandular tubes of the frontal horns), we 

 should note the muscles for moving the limbs, the thoraco-abdominal 

 appendages, and the alimentary canal. The latter consists of a short 

 oesophagus, a wider mid-gut, and a hind-gut opening through the 

 ■dorsal anal aperture. As to the presence of the antennal gland (in 

 the basal joint of the second pair of limbs) which occurs in the 

 Entomostracan Nauplii, nothing is as yet known. 



