viii ARTHKOPODA 283 



Scolopendra a segment is developed in front of the antennae 

 on which are a pair of rudimentary appendages, the prae-antennae 

 (pr.ant, Fig. 2'2^). No such segment and no such appendages have 

 been detected as yet in any other Arthropod, unless the rudimentary 

 antennae which have been described in the embryos of certain 

 spiders be homologous with them. 



It will be remembered that Kishinouye describes a pair of 

 coelomic sacs in the head region of Limidus and spiders ; now the 

 true prae-oral region or acron of Insecta never possess coelomic 

 sacs. It is therefore possible that these prae-oral coelomic sacs of 

 Arachnida may be the remnants of a lost prae-chelicerau segment 

 corresponding to the prae-autenna. 



The ectoderm covering each segment is thickened in two places 

 on each side, one immediately above the insertion of the limb which 



J fj 



Heymous regards as the rudiment of the tergum, and one below the 

 insertion of the limb which he regards as the sternum. The two 

 terga are separated by a thin dorsal membrane, and the two sterna 

 by a thin sternal membrane. This tritid division of the dorsal covering- 

 is indicated in a later stage, when chitin has been developed, by two 

 longitudinal furrows which mark out the dorsal sclerites into three 

 regions, and Heymons regards it as a reminiscence of a, TriloHte ancestry 

 for the Centipedes, and through them for the whole of the Insecta. 



(c) The nervous system develops as two bands of thickened 

 ectoderm underlying the coelomic sacs and separated by a median 

 groove. These bands are thickened in each segment, and these 

 thickenings constitute the ganglia. In each ganglion there is an 

 ectodermic pit, which owes its origin to the energy with which, at 

 these spots, cells are proliferated towards the interior to constitute 

 the nerve cells. The pits eventually become closed in and with- 

 drawn from the surface ; their outer walls do not become nerve cells 

 but form a sheath enveloping the ganglion. From the mid-ventral 

 membrane a string of cells is split off which, by contraction, gradually 

 draws right and left ganglia together. 



The brain consists of an archicerebrum, or original pair of supra- 

 oesophageal ganglia in the acron, and of two pairs of lateral pits, a 

 more median and a more lateral pair. These pits give rise to 

 ganglionic masses which increase the size of the original ganglion in 

 the acron. The small optic lobes are developed from the lateral pits. 

 The compound ganglion so formed is known as the protocerebrum. 

 The ganglia of the antennary region give rise to the deuterocerebrum, 

 those of the pre-antennary segment to the bridge connecting proto- 

 and deuterocerebrum. The ganglia of the intercalary segment give 

 rise to the tritocerebrum. Since the deuterocerebrum of Crustacea 

 resembles in its structure the deuterocerebrum of Insecta and Myria- 

 poda Heymons concludes that the two are homologous, that conse- 

 quently the antennae of Insecta correspond to the antennules "f 

 Crustacea, and that the prae-antennae of ,SVW^o/<//v/ and the segment 

 from which they spring have been entirely suppressed in Crustacea. 



