954 WORMS 



cells ' situated within the extremities of the pinnulated segments, 

 where they project inwards from the wall of the body ; these, when 

 set free, float in the fluid of the perivisceral cavity and multiply 

 themselves by self-division ; and it is only after their number has 

 thus been considerably augmented that they begin to increase in 

 size and to assume the characteristic appearance of ova. In this 

 stage they usually fill the perivisceral cavity, not only of the body, 

 but of its caudal extension, as shown at C ; and they escape from 

 it through transverse fissures which form in the outer wall of the 

 body at the third and fourth segments. The male reproductive 

 organs, on the other hand, are limited to the caudal prolongation, 

 where the sperm-cells are developed within the pinnulated append- 

 ages, as the germ-cells of the female are within the appendages of 

 the body. Instead of being set free, however, into the perivisceral 

 cavity, they are retained within a saccular envelope forming a testis 

 (A, a, a) which fills up the whole cavity of each appendage ; and 

 within this the spermatozoa may be observed, when mature, in 

 active movement. They make their escape externally by a passage 

 that seems to communicate with the smaller of the two just men- 

 tioned rosettes ; but they also appear to escape into the perivisceral 

 cavity by an aperture that forms itself when the spermatozoa are 

 mature. Whether the ova are fertilised while yet within the body 

 of the female by the entrance of spermatozoa through the ciliated 

 canals, or after they have made their escape from it, has not yet 

 been ascertained. Of the earliest stages of embryonic development 

 nothing whatever is yet known ; but it has been ascertained that 

 the animal passes through a larval form, which differs from the 

 adult not merely in the number of the segments of the body (which 

 successively augment by additions at the posterior extremity), but 

 also in that of the antennae. At G is represented the earliest larva 

 hitherto met with, enlarged as much as ten times in proportion to 

 the adult at B ; and here we see that the head is destitute of the 

 frontal horns, but carries a pair of setigerous antennae, a, a, behind 

 which there are five pairs of bifid appendages, 6, c, d, e,f, in the 

 first of which, 5, one of the pinnules is furnished with a seta. In 

 more advanced larvae having eight or ten segments this is developed 

 into a second pair of antennae resembling the first ; and the animal 

 in this stage has been described as a distinct species, T. quadricornis. 

 At a more advanced age, however, the second pair attains the 

 enormous development shown at B, and the first or larval antennae 

 disappear, the setigerous portions separating at a sort of joint (Gr, a, 

 ), whilst the basal projections are absorbed into the general wall 

 of the body. This beautiful creature has been met with on so many 

 parts of our coast that it cannot be considered at all uncommon, 

 and the microscopist can scarcely have a more pleasing object for 

 study. 1 Its elegant form, its crystal clearness, and its sprightly, 

 graceful movements render it attractive even to the unscientific 



1 See the memoirs of the Author and M. Claparede in vol. xxii. of the Linnean 

 Transactions and the authorities there referred to ; also a memoir by Dr. F. 

 Vdjdovsky in Zeitschrift f. Wiss. Zool. Bd. xxxi. 1878. 



