60 DRS. CARPENTER AND CLAPAREDE ON TOMOPTERIS ONISCIFORMIS. 



advanced stage of tlie life of Tomoj)teris than in the earlier (compare figs. 1 and 6 of 

 Plate LXII. vol. xxii.) ; and we have found that this difference results from a gradual 

 increase in their proportional length, which bears a constant relation to the stage of deve- 

 lopment attained by the body. In an example of this type which we believe to be one of 

 the earliest yet met with (fig. 14, PI. VII.), these second antennae {b, b) were very short, 

 and presented a much nearer resemljlance to the ordinary lateral appendages of the. body 

 than they do at a later period. But the distinction between T. onisciformis and T. qua- 

 dricornis is mainly rested by Drs. Leuckart and Pagenstecher upon the absence in the 

 former of the pair of cephalic appendages borne by the latter, between the " frontal horns " 

 and the " second antennse; " these, which were designated in the former memoir as the 

 " second pair of horn-like appendages," will now be spoken of by us as the first antenn'ce, 

 since it is clear that they have more relation to the pair of appendages immediately 

 behind them than to those in advance of them. Por, in common with the " second an- 

 tennge," they are setigerous, each of them bearing two setae within its terminal portion 

 (fig. 6) ; and in the early form just now alluded to, in which the " frontal hoi'ns " were 

 entirely wanting, these first antennse were of such a length as to be the principal append- 

 ages of the head («, a, fig. 14). A careful study of the successive stages of the develop- 

 ment of Tomopteris, in fact, makes it obvious that these first antennce are to be con- 

 sidered as larval, and the second as characteristic of the adult ; for the former progres- 

 sively diminish in relative size, and at last (in most cases) disappear altogether, whilst 

 the latter progressively increase both absolutely and relatively. In the stage represented 

 in fig. 1, Plate LXII. (vol. xxii.), which seems to correspond to the T. quadi'icornis of 

 Leuckart and Pagenstecher, the setigerous portion of the first antennae is separated from 

 the basal by a constriction which suggests the idea of an articulation (PI. VII. fig. 6. a) ; 

 about the time when the caudal appendage begins to be developed, the setigerous portion 

 usually detaches itself, while the basal part remains as a mere knolj or tubercle ; and in 

 the most advanced forms, this tubercle is commonly fou.nd to have entirely disappeared. 

 A vestige of it, however, is still to be seen in some specimens ; and it would appear, 

 not only from the observation made by one of us last year, but also from the mention 

 of these appendages by Mr. Huxley (p. 359), who described and figured them as " long, 

 curved, spine-like processes arising from the ventral surface of the narrow neck," that 

 these first antennae are occasionally retained even in the stage of advanced develop- 

 ment. It is obvious, therefore, that no distinction between Tomoiiteris onisciformis and 

 T. quadricornis can be drawn from the presence or absence of the first antennae ; and if 

 no other constant and important characters of difference can be specified than such as 

 clearly arise out of the respective ages of the specimens, the last-named species must be 

 abolished. 



In stating the other additional results of our observations, we shall for the most part 

 enumerate these, for the sake of convenience, in the order in which they occured to us in 

 proceeding from before backwards along the body of the animal. 



In the advanced specimens (by which we mean those in which the caudal prolongation 

 is beginning to show itself), the head bears on its dorsal surface a pair oi ciliated epaidettes 

 (PL VII. fig. 5. a, a), which extend over the edges of the bilobed nervous ganglion. These, 



