DEVELOPMENT OF PERIPATUS 



365 



of the anus. In the ejaculatory duct the spermatozoa, which are 

 thread-like, are made into spermatophores which are attached to 

 the female. It is uncertain how the spermatozoa get into the female. 

 Fertilisation is ovarian. 



While it is characteristic of Arthropods, in which chitin is so pre- 

 dominant, that ciliated epithelium is absent, it seems that in Peripatus, 

 which is much less chitinous than the others, ciliated cells occur in 

 some parts of the reproductive ducts. 



Development. — There is some variety of development in different 

 species. Thus there is much yolk in the ovum of P. novcB zealandicB, 

 extremely little in that of P. capensis. 



In P. capensis the " segmentation " is remarkable, for true cleavage of 

 cells does not occur. The fully " segmented " ovum does not exhibit 

 the usual cell limits. It is a protoplastic mass — or syncytium — with 

 many nuclei. Even when the body is formed, the continuity of cells 

 persists, nor does the adult lack traces of it. To Prof. Sedgwick this 

 singular fact suggested the theory that the Metazoa may have begun 

 as multinucleate Infusorian-like animals. 



The gut appears from a fusion of vacuoles within the multinucleated 

 mass, and a gastrula stage is thus established. The blastopore or 

 gastrula mouth closes except at the two ends, thus forming mouth 

 and anus. In its early phases the development resembles in many 

 ways that of an Annelid. But thereafter Arthropod characters super- 

 vene. Paired rudiments of appendages grow out (see Fig. 204, lower 

 right-hand figure), beginning with the antennae. The development of 

 the coelom becomes largely suppressed in favour of a quite different set 

 of cavities which become filled with blood and constitute the hcemocoele. 



In the ova of P. novce 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 

 crystallising out in situ from a mass of yolk, among which is a proto- 

 plasmic reticulum containing nuclei." 



Zoological position. — The synthetic characters of Peripatus and 

 its allies may be thus summarised : — 



Annelid Characteristics. 



Segment ally arranged nephridia 



as in Chaetopods. 

 The muscular ensheathing of the 



body. 

 The cilia in the genital ducts. 

 Less important are the stump-like 



hollow legs and the simple 



eyes. 



Arthropod and Tracheate 

 Characteristics. 



The presence of tracheae. 



The nature of the heart (a tube 

 with paired ostia communicat- 

 ing with a pericardium) and 

 the lacunar circulation. 



The modification of appendages 

 as mouth organs. 



The form of the salivary glands. 



The smallness of the genuine 

 coelom ; the cavity of the 

 body is haemocoelic. 



