SITUS INVERSUS IN ECHINOIDS 107 



case is, it seems to me, more or less related to, but distinct 

 in some respects from, the above-mentioned second category 

 (seep. 141). The idea came to my mind after the experiments 

 had come to an end, and it needs further test with special 

 reference to this question. 



The experiments were made during the early summer of this 

 year (1920) in the Zoological Department, Imperial College of 

 Science and Technology, London. It is my pleasant duty to 

 tender my hearty thanks to Professor E. W. MacBride 

 for his kind supervision and unceasing encouragement through- 

 out the time during which the work has been carried out. 

 The writing of the manuscript was done in the Natural 

 History Department of the British Museum. My cordial 

 gratitude is also due to Sir Sidney F. Harmer, Director 

 of the Department, for his kind permission to work there and 

 to use the library. 



2. Descriptions of the Larvae with Inverse Situs. 



It must at the outset be stated with regret that the descrip- 

 tions of internal structures as here given are founded on a very 

 few specimens which I could preserve and section. As will 

 be seen in the table (p. 115) the total number of reversed larvae 

 I found was more than 150, but with the hope of getting as 

 many metamorphosed young as possible I did not kill and 

 preserve many of them. The observation on the early stage 

 when the right hydrocoele makes its appearance, i.e. the 

 earhest visible sign of the abnormality, is also lacking. About 

 half a dozen metamorphosed young were obtained, but all 

 the rest died off gradually without affording me any oppor- 

 tunity of following the internal changes which had taken place. 



External Characters. — Eight larvae with the inverse 

 situs were first found on May 31, when they were eleven days 

 old. The ' larval ' body was quite normal both in size and 

 shape : two pairs of larval arms, post-oral and antero-lateral, 

 both symmetrical and fairly long ; postero-dorsal arms still 

 very short ; the posterior part of the body -rod beginning to 

 degenerate, with its club-shaped end separated from the rest 



NO. 261 I 



