DR. W. B. CARPENTER ON TOMOPTERIS ONISCIFORMIS. 357 



closely 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 

 cylindrical 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 theii' 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 ^Tinkling, 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 

 it is 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 there would be some 

 gradation from one type to the other ; so far is this, however, from being the case, that 

 the 17th pair (the^rs^ of the vermiform continuation) shows the most decided milikeness 

 to the 16th (the Icist of the anterior series) ; the 18th, 19th, 20th, and 21st are of nearly 

 the same size and aspect ; whilst the 22nd, 23rd, and 2Jith 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 so. 



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 was a larral 

 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 it 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 



