THE FORMATION OF THE ORGANS — THE NERVOUS SYSTEM. 239 



very difficult to examine. The larvae of Pauropus, like those of the Diplopoda, 

 have at first six pairs of limbs (Lubbock, Ryder, Latzel). * The second 

 and third pairs of legs are said to belong to one segment or rather donble 

 segment. The larva has onty a few segments, there being only one limbless 

 segment besides the anal segment. During the first moult two more pairs of 

 legs appear, and the larva then develops six, seven, eight, and filially nine pairs. 

 Still less is known of the development of Scolopendrella, although such 

 knowledge would be of great value,, seeing that special significance attaches 

 to this form. The youngest stage as yet observed had six pairs of legs, but 

 it is not impossible that this stage was preceded by others with fewer pairs 

 (Latzel). Each of the numerous moults that follow is accompanied by the 

 addition of a new pair of legs, until the full number of segments found in 

 the sexually mature animal is reached. According to Latzel, the course of 

 development of Scolopcndrdla strongly recalls that of the Polyxcnidae. 



3. The Formation of the Organs. 



Our knowledge of the formation of the organs in the Myriopoda 

 is still very incomplete. As far as we know, it appears to take place 

 in a somewhat similar way in the Chilopoda and the Diplopoda. 

 The statements made on this subject by Metschnikoff in his earlier 

 works have been followed by more detailed though not exhaustive 

 accounts by Sograff and Heathcote.j- 



The Nervous System. 

 All that we know of the supra-oesophageal ganglion is that it 

 has a paired origin in thickenings of the cephalic lobes which finally 

 separate from the ectoderm. At the time when the embryo of the 

 Jidus leaves the egg-shell, therefore long after the rudiment of the 

 brain has appeared, two pit-like depressions can be seen in the 

 cephalic lobes, resembling those met with in Peripatus, the Insecta 

 and the Arachnida (cf. Figs. 93, 94 C, 28 B, and 109 C). These 

 depressions are at first shallow, but deepen later, and sink into the 

 rudiment of the supra-oesophageal ganglion, the cells forming the 

 floors of the pits seeming to fuse with the cell-material of this latter. 

 The apertures of the pits narrow, and they become closed vesicles 

 (Heathcote). This formation of ectodermal vesicles in connection 

 with the rudiment of the brain specially recalls the condition of the 

 so-called ventral organ in Peripatus (Fig. 96 B, p. 190). The vesicles 

 finally disappear, and their significance has not been definitely 

 established, but we may, taking into account the conditions 

 prevailing in the Insecta and Arachnida (p. 62), suppose that 



* For more detailed statements on this subject we refer the reader to Latzel's 

 work (Xo. 10, Bd. ii., p. 21), where also the literature relating to it is quoted. 



t [See Heymon's more recent work on the Chilopoda, an abstract of which is 

 appended to p. 257. — En.] 



