PREFACE. 



THE present volume constitutes the final instalment of the 

 translation of the Lehrbuch der vergleichenden Entwicklungs- 

 geschichte der wirbellosen Thiere, and contains the Mollusca, 

 the Tunicata and the Cephalochorda. 



The immense strides which have been made during the last 

 eight years in the study of invertebrate embryology have 

 necessarily rendered much of this work out of date, and in no 

 part is this more apparent than in the chapter on the Tunicata, 

 and especially in the sections dealing with Salpa. Up to the 

 time of the appearance of this work in 1893, our knowledge 

 of the embryology of this genus was mainly based upon the 

 results of Salensky's investigations. His conclusions were so 

 startling that our authors expressed considerable doubt con- 

 cerning their accuracy, but, owing to the absence of any other 

 observations, were obliged to incorporate them in this work. 

 Since that time, the important investigations made by Brooks, 

 Heider and Korotneff have appeared, and these necessitate an 

 entire revision of the section dealing with the embryology of 

 Salpa. I, however, did not feel that it lay within my province 

 to rewrite this section, so I have contented myself with ap- 

 pending numerous footnotes pointing out wherein the recent 

 investigators differ in their observations and conclusions from 

 those cited in these pages. It is, however, impossible to do full 

 justice to this subject by means of footnotes, and the student 

 who desires to study the subject thoroughly is referred to the 

 original monographs. 



