MUSEUM OF COMPAEATIVE 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 v/hich Pilidmm 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 Pilidium could be com- 

 pared with those already known of Actinotrocha, befnTp, tbe arms had formed, 

 3ven 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 Pilidium 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 II. 



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 

 larvae 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 larvse are among the most common Annelid larvse 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 equatoriully 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 ilattened, 



* According to tlie last-mentioned authors, Loven's larva is the young of the 

 strange genus Po!i/r;ordiiis. 



t See Balfour, Treatise on Comparative Embryology. Our Polygordius larva is 

 like the European in having two ciliated belts. 



