reproduction and larval stages of "gefirei," 55 He established 

 the fact of dioecious Phasoolosoma and described the structure 

 of mature ova of Sipunculus nudus . Attempts at artificial 

 insemination of these ova were unsuccessful, and Krohn had 

 to be satisfied with the study of the larvae of Sipunculus 

 caught in plankton, whose description constituted the final 

 part of the work. Two small remarks by Krohn are concerned 

 with the vegetative reproduction of the annulated worm Syllis 

 and Autolytus . 56 He found in them this change of sexual and 

 asexual reproduction, which permitted comparison with true 

 alternation of generations. Many years later 57 Krohn again 

 turned to the study of reproduction in sillids, describing 

 the new viviparous species of polychaeta of this genus. 



Other of Krohn 's reports separate information about the 

 development of nemertineans, phronids, and chaetognaths . 

 The description of larvae and partial transformation of the 

 first two forms constitute the contents of a special 

 article^ (135) . On the question of the development of 

 nemertineans inside pilidium, Krohn inclined to the opinion 

 that pilidium is considered a helminth, giving origin to 

 worm-shaped sexual generation. Actinotrocha, in Krohn' s 

 opinion, is a larval stage of any worm, tentatively relating 

 to echiuroids. The process itself of the transformation of 

 actinotrocha he did not observe and noted only the disappear- 

 ance of the larval organs and the concentration of antennae 

 in the circumoral corona. 



In his excellent investigations on the structure of 

 chaetognaths, Krohn added the study of their development. 



55. A. Krohn, "Uber die Larve des Sipunoulus nudus nebst 

 vorausgeschikten Bemerkungen liber die Sexualverhaltnisse 

 der Sipunculiden," ARCH. ANAT . , PHYSIOL. (1851), 

 pp. 368 - 379. 



56. A. Krohn, "liber die Erscheinungen bei der Fortpflanzung 

 von Syllis prolifera und Autolytus proKfer," ARCH. 

 NATURG., 18, 1 (1852), pp. 66 - 76; "Uber die Sprossling 

 von Autolytus prolifer Gr.," ARCH. ANAT., PHYSIOL. 



(1855) , pp. 489 - 490. 



57. A. Krohn, "Uber eine lebendiggebarende Syllisart," ARCH. 

 NATURG., 35 (1869), pp. 197 - 200. 



58. A. Krohn, "Uber Pilidium und Actinotrocha," ARCH. ANAT., 

 PHYSIOL. (1858), pp. 289 - 301. 



572 



