XI 



PHYLUM ARTHROPODA 



613 



Arthropods, and presents some striking points of resemblance to 

 the Ctuetopoda. The development is in the main arthropodan, 

 especially as regards the mode of segmentation (at least in the 

 forms with much food-yolk, which are probably the more 



FIG. 485. Three somewhat later stages in the development of Peripatus capensis, showing 

 the mode of closure of the blastopore and the appearance of the primitive segments. A, stage 

 in which the blastopore (hi.) has the form of an elongated slit ; B, stage in which the blastopore 

 is closing up in its middle part ; C, stage in which the blastopore has become closed up except 

 the anterior part which has gone to form the mouth (mo.), and the posterior part which has 

 formed the anus (.tx.); the whole embryo has now become strongly curved towards the dorsal 

 side. (After Balfour.) 



primitive), the mode of closure of the blastopore, and of the 

 development of the mesodermal strands. Arthropodan also are 

 the relatively large size of the brain and the presence of tracheae, 

 the character of the heart with its pairs of ostia, together with 

 the clawed appendages, and the jaws in the form of modified 



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