CHAPTER XXVIII. 



GENERAL CONSIDERATIONS ON THE 

 ARTHROPODA. 



Ik reviewing once more the ontogeny of the various divisions of the 

 Arthropoda, we are specially struck by the uniformity of character 

 found among them. In the constitution of the eggs in the cleavage, 

 the method of formation of the germ-layers and the shape of the 

 embryo, there are so many points of resemblance that we are justified 

 by ontogeny in regarding the Arthropoda as phyletically distinct, 

 i.e., as forming a natural group, even though, as will be shown below, 

 the common stock divides near its root and gives rise to three great 

 branches known to us under the names of the Crustacea, the Arach- 

 nida and the Myriopoda-Insecta.* 



The eggs of the Arthropoda are as a rule distinguished by the 

 large quantity of yolk contained in them, and the equal distribution 

 of the latter (centrolecithal eggs of the Arthropoda). The typical 

 method of cleavage in the Arthropoda is a superficial one, which has 

 developed from total and equal cleavage, as may be seen from the 

 ontogeny of various Crustacea, f We also see that the Arthropodan 

 eggs, in those cases in which the food-yolk has secondarily degenerated, 

 undergo total cleavage (Clacodera, Peripatus Edwardsii,\ parasitic 

 Insects). In other cases the total cleavage perhaps still represents 

 a primitive condition, e.g., in Branchipus. In a few Arthropoda, 

 the egg appears to be telolecithal, and the cleavage is at first 

 restricted to only a small part of the egg (e.g., in Mysis, Cuma, 

 some Isopoda, and the Scorpiones). This apparently different method 

 of cleavage is, however, to be traced back to superficial cleavage. 



* [See Natural Science, Vol. x. "Are the Arthropoda a Natural Group?" — Ei>.] 

 f The statements in this chapter are based upon the facts already given in 



connection with the different divisions of the Arthropoda. The reader must 



refer for these to the preceding chapters. 

 J [See footnote, p. 165. — Ei>.] 



