CONTENTS OF PART FIRST 



PAGB 



Introduction 1 



Types of cleavage and gastralation 3 



Formation of the mesoderm 10 



Body cavity 11 



Chapter I. Porifera. By K. Heider 13 



I. Amphiblastula type 14 



II. Coeloblastula type 20 



III. Parenchymula type 22 



Development of Spongilla 25 



General considerations on the development and sys- 

 tematic position of the Porifera 28 



Origin of the different types of canal system . . 30 



Non-sexual reproduction of sponges .... 32 



Development of the gemmula 34 



Literature 36 



Chapter II. Cnidaria. By K. Heider 39 



Systematic 39 



I. Hydrozoa 39 



1. Hydroidea 39 



Development of metagenetic Medusae ... 40 



Budding of Hydroid polyps 43 



Development of the Medusa 43 



Comparison of polyp and Medusa. ... 45 



Budding and division in Medusae. ... 48 



Frustulation 49 



Development of polyps with sessile gonophores . 50 



Development of hypogenetic Medusae ... 53 



Developmental cycle of the Cuninas ... 58 



2. Siphonophora 59 



Systematic • . .59 



(a) Physophoridse 60 



Disconula larva of the Velellidse . . 65 

 Laws of budding in the Siphonophore 



stock 66 



(h) Calycophoridse 68 



General considerations on the origin of 



the Siphonophora 72 



ix 



