CHLOEON (EPHEMERA) DIMIDIATUM, 71 
` ldid not, in this state, see any facets to the eyes; but the posterior pair differ from the 
anterior three not only in size but in appearance, the posterior ones being brown, with 
indistinct traces of the separate eyes,—the ocelli, on the contrary, being composed of a 
greyish mass set in a black substance, 
The supporting hairs on the tarsi are five or six in number. 
The anterior gills have increased in size more than the others, and are now larger than 
the sixth pair. The posterior ones are still rudimentary. The first pair now commence 
to vibrate, but less continuously than the others, being often quiet when the five 
following pairs are in motion. The posterior gills differ from all the rest in possessing 
no power of vibration, either at this age or at any subsequent period. 
The caudal appendages are gg goths in length. The eighteen terminal segments are 
unaltered, except that the first five of them have coalesced, so that they can no longer be 
distinguished as separate segments. The basal portion now consists of seventeen seg- 
ments; so that there are thirty-five altogether. As before, the circle of spines is larger 
round the second, fourth, sixth, and eighth than on the other segments. The swimming- 
fringe commences on the eleventh segment, and is represented by a single hair on the 
inner side, and somewhat nearer the apex than the base. Some of the segments, how- 
ever, have two such hairs. 
The central caudal appendage is again about twice as long as before, and almost equals 
the basal segment of the lateral tails. It is, however, still unjointed. 
Sixth State (fig. 6). 
The animal moults again when it has attained the length of from 4*5 5ths to s4f;ths. 
The antenne have a length of 5 ths, and the third segment has again divided into 
three parts, of which the basal is the longest and the middle the shortest; so that, from 
the fourth to the twelfth inclusive, the segments increase in length as they diminish in 
diameter. If no other change had taken place, the total number of segments would, of 
course, under these circumstances, be twenty-two; but, practically, there are only eighteen, 
those four segments which were originally the fourth, fifth, sixth, and seventh having 
completely coalesced, and the same thing having taken place also with the two following. 
'The large posterior eyes now consist of a number of dark spots on a paler ground. 
The tarsi have seven supporting hairs. 
The lateral tails are Š% ths in length, and consist of twenty-six س‎ The three 
terminal segments have undergone little alteration, the two basal divisions of five seg- 
ments and the four basal segments of the third quintet (if 1 may use the expression) having 
respectively coalesced, so as to form now only four segments. This terminal portion of 
the tail, therefore, resembles the terminal part of the antennz in the gradual coalescence 7 
of originally distinct segments, and offers an additional resemblance in the remarkable 
fact that it has not at all increased in length. The basal segment, on the contrary, like | 
the third segment of the antennz, has rapidly developed itself. When the whole organ 
was ;2;ths in length, it measured gğpths; in this sixth state it has produced, directly 
or indirectly, nineteen new segments, which have a length of Zj5ths, and have therefore 
almost monopolized the whole increase. 
L2 
