AUTOLYTITS PKOLIFEE. 221 



in the ventral sac are advanced and some have escaped into the surrounding water. They 

 appear to have four or five post-cephalic segments. On the whole the figures are good. 



0. F. Miiller mentioned the male bud and the nurse-stock as separate species, but 

 he correctly figures them, and assigns them different habitats, the former being pelagic 

 with long bristles, the latter as only rarely occurring in the gulfs of Norway. He 

 describes the nurse-stock and its female buds as mother and foetuses, and he recognized 

 the ova in the buds. 



From this period onward the gradual unfolding of the life-history of Autolytus is one 

 of the most interesting chapters in zoology — associated as it is with one of the very 

 prominent examples of the so-called " alternation of generations." The nurse-stock and 

 its divergent sexual bucls were each regarded as separate species by the earlier observers, 

 such as 0. F. Miiller, (Ersted, and Max Miiller. It was not till Leuckart and Frey, and, 

 more definitely, Krohn, had shown the relationships of Sacconereis and Autolytus, and 

 De Quatref ages and he had studied the conditions in Syllis prolifera, that the true nature 

 of the processes were elucidated. Further advances were made by Alexander Agassiz, 

 Keferstein, Greef, Ehlers, Malaquin, and others. Thus the male and female buds, 

 formerly known as Polybostrichus and Sacconereis, were linked to the nurse-stock 

 Autolytus, and the structure and physiology of the parts elucidated. 



The genus Sacconereis was established by Johannes Miiller, 1 and his son Max Miiller 

 (1855) gave good descriptions and figures of both male and female of Sacconereis 

 helgolandica, the female carrying the ova in the sac and with swimming-bristles from the 

 fourth foot backward. He shows that the sperms are found in the first three bristled 

 segments of the male, and gives figures of four stages in the development of the young. 

 The forms were at that time considered as independent. 



Alex. Agassiz 3 gives a careful account of the life-history of a species he terms 

 Autolytus cornutus, which does not seem to differ in any essential respect from Autolytus 

 prolifer (and in this Greef and others coincide) except in the structure of the compound 

 bristles, which he figures as having a simple " sickle-shaped " tip, a feature at variance 

 with the usual structure of Autolytus, and which may have arisen from misinterpretation. 

 He furnishes an excellent description, with figures of the nurse-stock, male and female 

 bucls, and for the first time the development of the eggs (borne on the ventral surface in 

 a sac), and young issuing from these up to the stage of the nurse-stock, thus illustrating 

 the alternation of generations in the species. 



The Polybostrichus Mulleri of Keferstein 3 (1863) seems to be a male form with much 

 longer tentacles than usual, but it may be only a variety. The body is in two parts, the 

 anterior having 19 — 22 segments, the posterior with long swimming-bristles. 



Claparede 4 describes and figures a new species under the name of Autolytus roseus, 

 which bears a very close resemblance to a female Autolytus prolifer bearing the sac with 



1 "Ueber den allgemeinen Plan in der Entwickelung der Echinodermen/' 'Phys. Abh. Akad. 

 Wiss. Berlin/ 1852, p. 52 (1853). 



2 ' Journ. Bost. Soc. Nat. Hist./ vol. vii, p. 392. 



3 ( Zeitsch. f. w. Zool./ xii, p. 113, pi. xi, f. 1—6. 



4 < G-lanures/ p. 106, pi. vii, fig. 4 (1864). 



