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have to occupy a distinct position among the earlier Chaeto- 
pods, tracing its ancestry first to a progenitor of Parthenope, 
and through this to the stirps of iEolosoma and the Naids. 
Summary. — The following facts, with regard to the sexual 
form of Chcetogaster Litnncei, may be regarded as certainly 
ascertained. 
1st. The sexual form makes its appearance in October, 
and is theft but rare. It exhibits greater activity than the 
larvae. 
2nd. It is from twice to three times as large as the small 
larval chains of zooids, which measure from one tenth to one 
sixth of an inch in length. 
3rd. Sixteen or more segments bearing fasciculi of setae 
succeed in unbroken order to the head and cephalic fasciculi, 
forming a tapering individual worm, instead of three (tempo- 
rarily five sometimes) such segments in the larvae. 
4th. The alimentary canals of the adults and adolescent 
contain setae which have been swallowed. 
5th. The adolescent have often but half the number of setae 
prevalent in the larvae (twelve in cephalic and eight in 
abdominal fasciculi); the adult have invariably twice the 
larval number of setae in each fasciculus. 
6th. The first pair of abdominal fasciculi are situated more 
anteriorly than in the larvae, with whose first the second pair 
agree, not only in position, but probably homologically. 
7th. A thick cuticle exists, which is absent in the larva. 
8th. A general advance in the histological structure and 
specialisation of the viscera is apparent in the adult ; but 
the pharyngeal portion of the nervous system retains its 
simplicity. 
9th. There is a bilobed testis situate near the oesophagus, 
the spermatozoa from which float in masses in the perivisceral 
cavity when complete. 
10th. Ova develop posteriorly to the testes in paired, 
masses on the sides of the nerve-cord ; they also are detached 
and float in the perivisceral cavity. 
11th. Close to the first pair of abdominal fasciculi, at the 
inner margin of each, ai'e developed four stunted and thick 
bristles of generative function. 
12th. A clitellus surrounds the region of the worm about 
and between the two first pairs of abdominal fasciculi. 
13th. Some of these changes arise by modification in a 
worm already grown. How far any arise in the course of 
growth is not ascertained. 
Chcetogaster diaphanus. — In PI. XV, figs. 5, 6, I have 
sketched roughly the nervous system of this species. Leydig 
