2S6 PERIPATUS, MYRIOPODS, AND INSECTS. 



The gut appears as a large vacuole within the multinucleated mass, 

 and a gastrula stage is thus established. 



In the ova of P. novas zealandice, which have much yolk, a superficial 

 multiplication of nuclei forms a sort of blastoderm, which spreads over 

 almost the entire ovum. The segmentation in this case has been called 

 centrolecithal (the type characteristic of Arthropods), but it is again true 

 that for a long time the cells do not exist as well-defined units. It has 

 been said, indeed, that "the embryo is formed by a process of crystal- 

 lising out in situ from a mass of yolk, among which is a protoplasmic 

 reticulum containing nuclei." 



These examples may serve to show that the development of Peripatus 

 is veiy varied. 



Zoological position. — Professor Lang summarises the synthetic 

 characters of Peripatus as follows : — 



Annelid Characteristics. : Tracheate Characteristics. 



Segmentally arranged nephridia as ' The presence of tracheae. 



in Chaelopods. The nature of the heart and the 

 Segmentally arranged crural lacunar circulation. 



glands, like similar glands in The modification of appendages 



some Chsetopods. as mouth organs. 



The muscular ensheathing of the The form of the salivary-glands. 



body. The smallness of the genuine body 

 Less important are the stump- cavity or ccelom. 



like legs and the simple eyes. 



The ladder-like character of the ventral nervous system (cf. primitive 

 Molluscs, Phyllopod Crustaceans, and Nemerteans) is probably primitive. 

 That salivary glands and genital ducts are homologous with nephridia, is 

 a fact of much morphological interest. It is possible that the slime 

 glands are modifications of crural glands, and that the latter are 

 homologous with the parapodial glands of some Annelids. It is not 

 certain that the antennae, jaws, and oral papillae of Peripatus precisely 

 correspond to the antenna, mandibles, and first maxillae of Insects. 



Our general conclusion is that Peripatus is an archaic type, a survivor 

 of forms which were ancestral to Tracheata and closely related to 

 Annelids. 



Second Class of Tracheata Antennata. — Myriopoda. 

 Centipedes and Millipedes. 



These animals retain a worm-like shape ; the numerous 

 rings of the body and the appendages they bear are very 

 uniform ; there is little division of labour. Simple wingless 

 insects, known as Collembola and Thysanura, are closely 

 approached by such Myriopods as Scolopendrella ; and it is 

 likely that Myriopods and Insects are divergent branches 

 from a common stock. 



