xi PHYLUM ARTHROPODA 597 



is established between the embryo and a modified area of the 

 uterine epithelium, the placenta thus formed evidently providing, 

 like the placenta of Mammals, for the nourishment of the embryo. 



In P. capensis (Fig. 492) proliferation of cells gives rise to an 

 oval thickening behind the elongated blastopore. The mesoderm 

 takes its origin at this point and extends forwards in the form of 

 two germinal bands, one on the right of the blastopore and the 

 other on the left. These bands undergo a division into rudiments 

 of segments the division beginning in front. The lips of the 

 blastopore meanwhile become approximated, and fuse throughout 

 the greater part of their length, leaving only an anterior and a 

 posterior opening ; these go to form the mouth and the anus 

 respectively. The division into segments soon becomes well 

 marked. The cavities in the mesoderm of the segments give rise 

 only to the internal vesicles of the nephridia and the generative 

 ducts, which thus alone represent the ccelome. The nephridia 

 themselves are developed as ingrowths of the ectoderm. At 

 the anterior end the head lobes become distinguishable. The body 

 elongates, and the head and trunk become differentiated. The 

 limbs now arise as ventro-lateral outgrowths which are developed 

 from before backwards. The jaws originate in the same manner 

 as the limbs, as external projections, and only later become enclosed 

 by folds that give rise to the buccal cavity of the adult. 



Distribution. The various species of Peripatus are all terres- 

 trial, and are found in damp localities, under bark, or dead timber, 

 or stones. Some twenty-nine species occur in the Neotropical 

 region ; one in South America ; eight in Africa ; four in Malaya ; 

 one in New Britain, and eight in Australasia. 



Relationships. Peripatus is the most primitive of existing 

 Arthropods, and presents some striking points of resemblance to 

 the Chaetopoda. 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 

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

 development of the germinal bands. 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 limbs. 

 The nephridia, on the other hand, and their modification in certain 

 segments to form the gonoducts, which are ciliated internally, 

 are annulate in character ; and in all probability the slime glands 

 and coxal glands correspond to the setigerous sacs of the Chsetopoda. 

 The nervous system is peculiar, and is most nearly paralleled among 

 the Platyhelminthes and the Mollusca. Also peculiar, and serving 

 to distinguish Peripatus from the rest of the Arthropoda, are the 

 large number of stigmata and their irregular arrangement, the 

 presence of only a single pair of jaws, and the nature of the cuticle. 



VOL. I Q Q 



