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MUSEUM OF COMPARATIVE ZOOLOGY. 195 
Phoronis. Indeed, I have borrowed from those who have written on the 
metamorphosis of Actinotrocha the term “critical stage,” on account of this 
likeness. I have no hesitation now in identifying Pilidium recurvatum as a 
young Nemertean, rather than a young Gephyrean, although I am doubtful 
whether I know the generic name of its parent. 
Prominent among the characteristics which Pilidiwm recurvatum shares with 
Actinotrocha is the existence in both of a belt of cilia which divides the body 
into such unequal parts. These regions thus marked out resemble each other 
in general shape, and probably, if a younger larva of Pilidiwm could be com- 
pared with those already known of Actinotrocha, before the arms had formed, 
even closer resemblances might be traced between them. Whatever likenesses 
there are earlier in the internal organs, they have now been completely masked 
by the progress of the development. It is very difficult to compare the 
youngest known Pilidiwm recurvatum with Actinotrocha, as far as the internal 
organs are concerned, notwithstanding there is such a similarity in external 
outlines. 
Polygordius (‘‘Loven’s Larva”). 
Plate Il. 
The writings of A. Agassiz, Schneider, and Hatschek,* on the develop- 
ment of the very common larva known as Loven’s larva, have given a history 
of its metamorphosis from a somewhat advanced larva into the adult form. 
All is still dark, however, in regard to the segmentation of the egg and the 
earliest forms which the larva passes through. As any contribution to either 
of these parts of the subject must have a value, figures and descriptions of two 
larve younger than any of this worm yet described are here introduced. These 
take us one step nearer a complete knowledge of the growth and early history 
of this most interesting worm. 
Loven’s larvee are among the most common Annelid larve taken in the dip- 
net at Newport. They are found in all conditions and of all sizes, sometimes 
swarming in numbers in the collecting glasses. The youngest larva which was 
found, Fig. 10, has a general structure as follows. 
The body of the larva is spherical, transparent, and as gelatinous as that of a 
Medusa. In its movements in the water it tumbles about, moved principally 
by the strokes upon the water of an equatorially placed double belt of cilia. 
The larva has a slightly irregular spherical form, rounded above, somewhat 
flattened below, and girt midway by two f rings of motor cilia. The hemi- 
sphere above these ciliated bands may be called the upper hemisphere, and that 
below the lower. The upper hemisphere is, with the exception of a slight prom- 
inence on one side, regularly rounded and dome-shaped ; the lower is flattened, 
* According to the last-mentioned authors, Loven’s larva is the young of the 
strange genus Polygordius. 
t See Balfour, Treatise on Comparative Embryology. Our Polygordius larva is 
like the European in having two ciliated belts. 
