244 Mr. A. Agassiz on the Young Stages 



Buscli and by Frey and Lsuckart*, the adults of which arc not 

 known, we must be exceedingly careful in our identilications of 

 apparently closely related forms, and give these identifications 

 more as hints for future observers than as positive statements. 



The young of Pohjdora and Neiine, like the young of Leuco- 

 dora, Claparede, are kept in confinement with the greatest 

 ease; hence the possibility of tracing the changes of growth in 

 a connected manner until they have assumed unmistakeably the 

 features and habits of the adult, and built their cases on the 

 bottom of the jars where they are confined. The youngest 

 stages observed (PL V. fig. 26) are considerably more advanced 

 than those of Claparede, having already lost, if they ever pos- 

 sessed them, the bunches of ringed bristles so characteristic of 

 the younger stages of many Annelids, such as Leucodora, Clap., 

 Nerine, and Spio, The tentacles of the head are developing; 

 and there remain but slight traces round the head and anus of 

 the former rings of vibratile cilia, as well as very narrow bands 

 of short vibratde cilia on the dorsal side; similar bands are 

 found on the lower side, composed of larger cilia, which greatly 

 assist in locomotion. These bands are less powerful towards 

 the extremities, being greatly developed towards the middle, 

 especially on the lower side. The vibratile ring surrounding 

 the anus is less prominent than in Claparede's embryo; the 

 anterior and posterior rings of cilia, as well as the transverse 

 bands, diminish rapidly in size with advancing age, so much so 

 that in fig. 28 they have almost totally disappeared. In the 

 youngest stage seen (fig. 26) there are four well-developed an- 

 terior rings, each provided with an up])er and lower bunch of 

 bristles, the dorsal bunches being the longest, and diminishing in 

 length as they recede from the head, the lower bunches consisting 

 of bristles all of the same length ; the fifth ring [r^ is much wider 

 than any other, and has only three short, stout bristles on each side; 

 then comes the sixth ring, each side having a bundle of bristles 

 similar to those of the smaller cluster of the four anterior rings; 

 in the subsequent rings, which are nearly equally developed, 

 having a slight lateral swelling and rudimentary dorsal cirrus, 

 we find a similar bundle of bristles, and, in addition, in the 

 seventh, eighth, and ninth, a single hook-shaped bristle ; while 

 immediately in front of the anal extremity the rudimentary 

 rings have merely a couple of thin bristles. The three rings 

 following the sixth have rudimentary glands (g), first observed 

 in this genus by Claparede in his description of Pohjdora ; they 

 consist, however, of a smaller number of glands, only two or 

 three in each bag. The general colour of Polydora at this period 



* *Beitrage zur Kenntniss wirbelloser Thiere' (Braunschweig, 1847), 

 pi. l.fig. 19, p.98. 



