DEVELOPMENT OF PERIPATUS. 



25s 



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 coelome is represented 

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

 series of truly coelomic 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 ectodermic invagination forming 

 pharynx and gullet (fore-gut or stomatodseum), and of a short posterior 

 ectodermic invagination forming the rectum (hind-gut or proctodseum). 



The nephridia have a two-fold 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 epitheUum of a persistent portion 

 of the true coelome or primitive body-cavity. 



Zoological Position of Peripatus. I shall summarise what 

 Lang says on this subject in his work on Comparative 

 Anatomy. 



Annelid Characteristics, Tracheate Characteristics, 



of Peripatus. 



Segmentally arranged nephridia as 

 in Chsetopods. 



Segmentally arranged coxal glands, 

 like similar glands in some 

 Chsetopods. 



The muscular ensheathing of the 

 body. 



Less important are the stump- 

 like legs and the simple 

 eyes. . 



The presence of trachece. 



The nature of the heart and the 



lacunar circulation. 

 The modification of appendages 



as mouth-organs. 

 The form of the salivary glands. 

 The smallness of the genuine 



body-cavity or coelome. 



