244 INVEETEBEATA chap. 



egg into blastomeres persists ; segmentation is represented by multi- 

 plication of nuclei and their arrangement at the surface of the egg. 

 The first differentiation of layers takes place in connection with a 

 ventral thickening of the blastoderm, which may be regarded as 

 representing the blastopore of Peripatus. The mesoderm soon 

 becomes split into two bands, right and left, and in each of these 

 a series of coelomic pockets makes their appearance. 



It is difficult to decide whether Kishinouye is right in asserting 

 that the first pair of these pockets is found in the cephalic lobes, and 

 is distinct from the pair which appears later in the segment bearing 

 the chelicerae, or whether Brauer is right in asserting that the coelom 

 of the cephalic lobe is a mere forward production of the coelom of the 

 cheliceral segment. 



If Kishinouye is right it is quite probable that the coelomic sacs 

 in the cephalic lobe represent a lost anterior segment in these 

 animals, a segment which is in front of the segment corresponding to 

 the first antennae of Peripatus, centipedes, and insects. As we 

 shall see later, this lost segment is distinctly represented and bears 

 vestigial appendages in the embryo of the centipede. We may 

 provisionally accept Kishinouye's view, remarking merely that 

 it seems clear that most interesting results would be obtained by a 

 revision of his work with the aid of modern methods, when many of 

 these vexed questions might be solved. 



We form, therefore, the following picture to ourselves of the 

 manner in which Arachnida were developed. They arose from 

 ancestors in which all the segments except the first bore bifurcated 

 appendages, with plate-like exopodites and more or less leg-like 

 endopodites. The first pair of appendages, however, had been 

 modified into antennae, or tactile organs, and were subsequently lost. 

 The succeeding appendages had their endopodites modified into 

 walking and grasping organs and had lost the exopodites, whilst the 

 hindermost appendages retained their plate-like form and assumed 

 respiratory functions. Such ancestors must have closely resembled 

 Trilobites, but their divergence from that group consisted in this, 

 that in Trilobites, and to a still greater extent in Crustacea, the 

 appendages immediately following the antenna tended to become 

 gnathites (or jaws), by the diminution in size of their distal members 

 and the development on their proximal members of cutting blades ; 

 and this process went only a very small way in Arachnida, in which 

 the corresponding appendages functioned as the main organs of 

 locomotion. 



The plate-like form of limb is no doubt the original form in 

 Arachnida as it is in Crustacea ; and the process of transforming 

 these plates into rounded legs, to which the differentiation of the 

 front part of the body of an Arachnid is due, began in front and 

 travelled backwards. Pantopoda probably represent a group in which 

 it went farther back than in Arachnida, and in which the hindermost 

 appendages were lost. 



