214 Mr. A. Agassiz on the Young Stages 



quarter of an inch ; the subsequent changes are principally 

 limited to alterations in the shape of the head and the gradual 

 disappearance of the articulations, the only trace of them left 

 being the corresponding convolutions of the digestive cavity. 

 The oral and anal vibratile cilia disappear rapidly (figs. 12 & 13); 

 the head becomes more rounded ; the antennae, having attained 

 their maximum size (figs. 12 & 13), grow less and less promi- 

 minent and rapidly vanish ; so that the head of the young worm 

 has now the shape of fig. 14, which was its condition four 

 months after the stage represented in fig. 11. The articula- 

 tions have become obliterated; no trace can be found of the pig- 

 ment spots, which have gradually grown smaller and less nu- 

 merous; and the young worm in its motions and attitudes re- 

 minds us strongly of Nemertes and the like Annulata. About 

 a month later the head is even less prominent, and is separated 

 from the body by the characteristic neck of the Nemerteans, 

 the tentacles having altogether gone, the only trace of them 

 being very slight swellings on each side of the head. The 

 young worm loses at the same time its cylindrical shape, and 

 in fig. 14 has already become greatly flattened. This is quite 

 well shown in fig. 16, a profile view of fig. 15. The young Ne- 

 mertean is now nearly half an inch long, and is usually found 

 slightly coiled on the bottom of the jar in which it is kept ; on 

 being disturbed their motions are somewhat like those of the 

 Nemerteans. The posterior extremity is much smaller than 

 the anterior, the width of the worm increasing towards the 

 head. As it grows older this diff*erence is lost, the head be- 

 comes still less prominent, and finally, as in fig. 17, when the 

 young worm is five months older than fig. 11, the width of the 

 head is less than that of the body, and the eyes have moved 

 nearer the neck. 



There is but little doubt, from the foregoing observations, 

 that Loven's larva becomes eventually a Nemertean closely 

 allied to Polia ; my oldest larvae, however, were far from being 

 adults, and their generic affinities cannot be more closely inti- 

 mated at present. There is little exceptional in the develop- 

 ment of the larva from that of the other Annelids, as has been 

 maintained; and, like other Annelids, it early assumes the features 

 of the adult; and new rings are developed next to the anal ring, 

 in accordance with the observations of all writers on the subject. 



Spirorbis spirillum, Gould (non Pagenst., an Lam. ?). 



The history of the development of Spirorbis has been given 

 in full by Pagenstecher* ; I bring up the subject here to show 



* " Untersuchungen iiher niedere Seethiere aus Cette. Entwickelungs- 

 geschichte u. Brutpflege v. Spirorbis spirillum" in Zeits. f. wiss. Zool. 

 1863, xU. p. 487, pis. 38 & 39. 



