290 PERIPATUS, MYRIOPODS, AND INSECTS. 



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

 this singular fact suggested the theory that the Metazoa may have begun 

 as multinucleate Infusorian-like animals. 



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

 and a gastrula stage is thus established. 



In the ova of P. nova sealandia, 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 crystallis- 

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

 reticulum containing nuclei." 



From these examples the student will perceive how difficult it is to 

 give a succinct account of the development of Peripatus. 



Development of Organs. 



The hypodermis is ectodermic, the cuticle an external product 

 thereof. 



The muscles are as usual derived from the mesoderm, which arises 

 from two ventral mesodermic strands. These are subsequently divided 

 into hollow segments. The true body cavity or ccelome is represented 

 by the original cavities of the mesoderm segments. In the adult this 

 series of truly ccelomic cavities is hardly represented except by the inner- 

 most portions of the nephridia. The apparent body cavity is a secondary 

 cavity, consisting, for the most part, of blood carrying or vascular 

 spaces, subsequently established in the mesoderm. It is divided into 

 five regions, the central space, the two lateral cavities, and the cavities 

 of the legs. 



The appendages are outgrowths of the body wall. They, and all the 

 segmentally arranged parts, develop progressively from in front back- 

 wards. 



The nervous system is derived from ectodermic thickenings which 

 sink inwards. It develops from in front of the mouth backwards. 



The food canal consists of the long endodermic mid gut or mesenteron 

 (the gastrula cavity), of an anterior ectodemiic invagination form- 

 ing pharynx and gullet (fore gut or stomatoda;um), and of a short 

 posterior ectodermic invagination forming the rectum (hind gut or 

 proctodieum). 



The nephridia have a twofold origin. The internal funnel is derived 

 directly from part of a mesodermic segment or vesicle. The rest of the 

 nephridium is invaginated from the ectoderm. 



The reproductive organs arise on the epithelium of a persistent portion 

 of the true ccelome or primitive body cavity. 



Zoological Position of Peripatus. — Professor Lang, in his work on 

 Comparative Anatomy, summarises the synthetic characters of Peripatus 

 as follows : — 



