BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 369 



Numerous specimens of Polia involuta, Van Ben., were sent from St Andrews 

 in April, loaded with ova, and their development could easily be followed out. 

 The newly deposited eggs (Plate XIV. fig. 1), are somewhat ovoid, about ^oth of 

 an inch in their long and ^-©th to gihjth in their short diameter, and appear to 

 possess only a single investment. They are not simply enclosed in a sheath, as 

 M. Van Beneden says, but the animal, during deposition, envelopes them and its 

 body in a tough hyaline mucus, afterwards withdrawing itself therefrom, as in 

 Borlasia, so that the whole forms a tunnel of mucus, with the ova in its 

 walls. The spiral condition of some of the masses was due to the coiled condi- 

 tion of the animal during deposition. After extrusion the ova pass through the 

 usual stages, and the embryo in each is sometimes ciliated on the tenth day 

 (Plate XIV. fig. 2), although entire dependence cannot be placed on this date, 

 since development occurs within as well as without the body of the parent. In 

 a short time the young are extruded either with a pair of eye-specks, or without 

 them, and furnished with a very long anterior, and a shorter posterior ciliary tuft 

 or whip (Plate XIV. fig. 6). Moreover, numerous adult specimens are found 

 towards the end of April to contain ova with ciliated young, showing that im- 

 pregnation, as may easily be understood, can take place through the genital 

 pores. In many of the ova the embryo had two reddish eyes, and some were 

 extruded from the body of the parent in a free state, so that they sailed about 

 actively through the water as ciliated pyriform bodies. The ciliation of the 

 oesophageal region in those with the eyes was very distinct ; indeed, after the other 

 and apparently more delicate tissues of the animal had become disintegrated, this 

 region was left in active ciliation— dissected out, as it were, by rapid decay. 

 This somewhat globular oesophageal region has probably been mistaken by M. 

 Van Beneden for a mouth. The same author fell into the error of supposing 

 that a form having a smooth outline was developed within its progenitor with 

 the long ciliary tuft, the former representing the scolez, and the latter the 

 proglottis; in short, as he says, a case of digenesis, and not a metamorphosis. But 

 his figure* represents the so-called proglottis as furnished with two eyes exactly 

 in the same manner as the scolex, yet he neither mentions having seen the one 

 form inside the other, nor figures this interesting condition. No such mode of 

 development has ever been seen by me, either in the case of those ova deposited in 

 the unimpregnated condition, or in those developed within the body of the parent ; 

 but the same gradual changes ensue in the young of this animal as in Tetras- 

 temma, and, as will afterwards be seen, also in Cephalothrix. 



Many of the parent-specimens having developing young in their interior are 

 feeble, and almost in a decaying condition inside their sheaths, so that their inert 

 bodies seem but the nidi for the growth of their progeny, each of which, pro- 



* Op. clt. pi. iii. fig. 28. 

 VOL. XXV. PART II. 5 B 



