DEVELOPMENT OF THE PECT1NIBRANCHIATA. 349 



we observe, for the first time, a contractile vesicle similar to that 

 which exists in Buccinum undatum. 



After the lapse of eight weeks the young had not yet left the 

 capsule, but when one was taken out, it began to creep like the 

 adult animal, with foot, tentacles and siphon stretched out. 

 It is then distinguished from the adult by the incomplete dis- 

 appearance of the lobes, by the shell being still soft, and by the 

 spire having only one or two turns (fig. 6). From the ninth 

 and tenth weeks the young begin to leave the capsules, the 

 rounded lobes disappear, and behind the tentacles an elevated 

 line may be seen occupying their previous place (fig. 7) ; the 

 shell has elongated and approximates more nearly that of the 

 adult; it is hard, frangible, and almost opake; the last turn of 

 the spire, however, is not yet formed. 



We shall not describe the development of the organs of 

 Purpura at greater length, because it does not differ from what 

 we see in Buccinum undatum. 



We may call the attention of the reader to the interesting 

 investigations of Kolliker and Siebold* on Actinophrys Sol and 

 on Diplozoon paradoxum; for, perhaps, something approxima- 

 ting to what we have just described may be found in their ob- 

 servations. 



In conclusion, we may add, that we entertain a strong desire 

 to have the opportunity of continuing our investigations upon 

 other genera allied to Buccinum and Purpura, for assuredly, 

 with some slight modifications, all these Mollusks are developed 

 in the same \vay. 



Summary. 



For the more ready comprehension of the history of the de- 

 velopment of Buccinum undatum and of Purpura lapillus, we 

 here shortly sketch the most essential points. 



Buccinum undatum. 



1 . The ovigerous capsule is filled with a transparent, colour- 

 less viscous liquid which resembles white of egg. Each capsule 

 encloses a mass of ova (of 6-8 centimetres). 



* Z I'it schr'ift fiir wiss. Zoologie, torn. i. p. 198, and torn. iii. p. 62. 



