THE ARTHROPODA. 413 



the Arachnida. In the latter, the mesoderm runs forward from the 

 point of origin in the form of two bands (mesoderm-bands) on either 

 side of the middle line. These two hands are also found in 

 !'• ripatus, the Myriopoda, and the Insecta, as well as apparently 

 in the Pantopoda, while, in the Crustacea, the arrangement of the 

 mesoderm is less regular. Some Crustacea, however (Branchipus, 

 Cymothoe), show a similar regular form of mesoderm-rudiment. 



The paired rudiment of the mesoderm breaks up into segmental 

 divisions in a somewhat similar way in all Arthropoda. These 

 divisions are the primitive segments (mesodermal somites), which 

 either become hollow, and are then known as coelomic sacs, or are 

 not thus modified, but soon break up into mesenchymatous tissue. 

 This latter is the case in most Crustacea, in which coelomic sacs are 

 rarely to be found, but the former condition occurs in the Xiphosura, 

 Arachnida, Pantopoda, Onychophora, Myriopoda, and Insecta. 



Although the primitive segments have as a rule a very similar fate, 

 and undergo similar modifications in all Arthropoda, certain differ- 

 ences are to be found in the various classes in the size attained by 

 them, and in the time at which their further differentiation begins. 

 The most primitive condition is exhibited in Peripatus, in which the 

 primitive segments in their large size resemble those of the Annelida 

 (Fig. 100, p. 200). The Myriopoda and the Orthoptera follow next 

 in the conspicuous development of the primitive segments within 

 the germ-band (Figs. 168 and 169 A, p. 343), while in the other 

 Insecta the coelomic sacs are from the first small, a considerable part 

 of the mesoderm being, as a rule, excluded from participation in the 

 formation of these sacs (Fig. 158, p. 321). In the Crustacea, the 

 development of the coelomic sacs is almost entirely suppressed. The 

 Arachnida, on the contrary, which in many other respects appear as 

 a modified group, are distinguished by the fact that in them the 

 coelomic sacs are unusually large, and even in the later stages of 

 embryonic development (at the time when the heart is forming) 

 extend almost to the dorsal middle line (Fig. 45 and 46, p. 88). 



The appearance and further development of the organs in the 

 different groups of the Arthropoda show remarkable and important 

 agreement. 



In the case of the nervous system it ha- been proved that an 

 invaginate middle strand and two lateral strands almost universally 

 take part in the formation of the ventral chain of ganglia. The 

 fibrous substance appears on the inner surface of the ganglionic rudi- 

 ments, and is only later taken into the latter, a process which must 



