624 FE. H. GRAVELY. 
characteristic of the ancestral stock of the order, for whilst 
natural selection was acting on the setze of worms that had 
finished their larval pelagic life and taken to the bottom, it 
must also have been acting in another way on those of the larve ; 
and the differentiation of the two distinct types may be due 
to both these causes equally. Agassiz, indeed, says (1867, 
p. 254): “ The presence of temporary bristles of huge size in 
the young of so many Annelids is a feature of the greatest 
interest from a paleontological point of view. We find 
repeated in Annelids the same striking coincidence between 
certain features only embryonic in the present types and 
characters of the adults in past geological time. I was par- 
ticularly struck with this coincidence when examining a series 
of drawings of fossil] Annelids kindly shown me by Mr. O. C. 
Marsh, of New Haven, which were all provided with bunches 
or single bristles of these large, rough sete, entirely out of 
proportion to the width of the body, and similar to those 
found in the embryonic Annelids we have noticed.” But 
there appears to have been no confirmation of this important 
point since its publication. 
Other larval characteristics found in two families (the 
Spionide and the Polydoride) of the Spioniformia are the 
presence of decidedly specialised interparatrochs and of a 
vestibule from which the true mouth opens into the cesophagus 
as described above. In larve of the Chetopteride and 
Magelonide this vestibule is not found, but their gaping 
funnel-like mouths (for the early development of Chetop- 
terus see Wilson, 1882, pp. 283-286, Pl. XXII, figs. 63-84, 
and Pl. XXIII, figs. 6-8, and for the later development of the 
same genus see Béraneck, 1894; for the early stage that alone 
has a funnel-like mouth in Magelona see Claparéde, 1863, 
pp. 74-75, Pl. X, fig. 9) may easily have resulted from an 
exaggeration of the vestibule as found in other Spioniform 
larvee. 
So little is known of the larva of Magelona before the 
complete loss of the cilia and funnel-like form of the 
mouth (Claparéde appears to be the only investigator who 
