DR. W. B. CARPENTER ON TOMOPTERIS ONISCIFORMIS. | 357 
elosely resembles that which is seen in the narrow canal of the earlier phase when partially 
. distended with fluid. | * 
The posterior portion of the body, however, constituting about two-fifths of its entire 
length, presents a number of very marked differences in conformation. Its aspect is more 
eylindrical and worm-like; its parietes are relatively thicker and stronger, and their 
. transverse wrinklings far more decided. It bears eight pairs of slightly-developed ap- 
pendages, which might at first sight be supposed (especially in regard to their terminal 
bifurcation) to be rudimentary forms of the fin-feet of the anterior part of the body, but 
which a more careful examination will show to be formed upon a different plan; for 
whilst the appendages that bear the fin-feet, as already described, contain extensions of 
the general cavity of the body, the parietes of which correspond with those of the body 
itself both in their tenuity and in their delicate transverse wrinkling, those of this vermi- 
form continuation are solid tubercles of ovoidal shape, put forth from the comparatively 
thick ‘integument, and which do not receive any prolongation of the perivisceral cavity. 
On reverting to figs. 1 and 5, it will be seen that in that period of the animal's life at which 
itis most rapidly undergoing increase upon the ordinary type, the last and consequently 
least developed pair of appendages is still formed upon the same plan with those anterior 
to it; and the same is the case with the 16th pair in the more advanced form we are now 
considering. If the 17th and seven succeeding pairs appertaining to this vermiform con- 
tinuation had any relation to the preceding, we should expect that tliere would be some 
gradation from one type to the other; so far is this, however, from being the case, that 
the 17th pair (the first of the vermiform continuation) shows the most decided unlikeness 
fo the 16th (the last of the anterior series) ; the 18th, 19th, 20th, and 21st are of nearly 
the same size and aspect; whilst the 22nd, 23rd, and 24th become progressively smaller, 
the last pair being scarcely distinguishable. But further, at the very same point at which 
this change manifests itself in the condition of the body and of the appendages, there is a 
very marked change in the condition of the intestinal canal; for this, instead of being | 
continued upon the straight and simple plan which so remarkably characterizes it in the 
anterior portion of the body, abruptly begins to present the appearance represented in 
figs. 6,7. Whether this appearance is due to convolution or to sacculation, I am unable 
to state with certainty. I was unwilling to submit my only specimen to the compression 
Which would have been requisite to ascertain this point, until I had secured a good | 
` delineation of it; and as it died and began rapidly to disintegrate before I could subject — 
it to further investigation, I was deprived of the opportunity of subsequently doing 80. 
The extreme delicacy of structure and the low grade of development of the earlier 
phase of this interesting creature having strongly suggested to me that it wasa larval 
form, the resemblance presented by the vermiform continuation, in the more advanced 
specimen, to the ordinary Annelidan type, together with its higher development and its 
appearance of greater permanence, led me to conjecture that this posterior portion would 
become the true type of the species, the anterior probably separating itself from Jt So 
soon as it should have acquired the power of sustaining itself independently, after the 
fashion of the Bipinnaria-zooid of the Star-fish. And notwithstanding that the informa- 
tion I have subsequently received as to the phenomena witnessed by other observers 
