THE ENTEROPNEUSTA. 659 
only the merest traces of a lumen. As is often the case with the collar cord, when the 
lumen shews a marked tendency to occlusion, the dorso-ventral diameter as seen in transverse 
section tends to become relatively great as compared with the transverse diameter (Pl. XLIV. 
fig. 29). The number of the dorsal roots is very variable. There may be as few as one, 
though four seems to be the maximum. 
The cornua of the nuchal skeleton are on the whole long for members of this genus, 
but shew a considerable amount of variation. They may extend over only 4 of the collar 
length, or their length may be equal nearly to } that of the collar. 
Trunk. The branchial portion of the cesophagus is of about the same size as_ the 
ventral part. The relative length of the branchial region as compared with the collar length 
is short, it being seldom more than half as long again as the latter and sometimes even 
less. In the large specimen figured on Pl. XXXVIL. fig. 9, however, it was rather more 
than three times the collar length. 
The post-branchial groove on the other hand is longer than usual, averaging between 
and 4 the length of the branchial region. 
on 
The genital folds are somewhat small (Pl. XLVI. figs. 52 and 56) and become mere 
ridges in the region of the post-branchial groove (Pl. XLVI. fig. 58). 
SOME ANATOMICAL POINTS. 
As Spengel in his latest pubheation (08) differs on one or two points from Willey 
it may be worth devoting a few lines to them. In two of these cases, the pygochord and 
the lateral septa, I find myself in agreement with Spengel rather than Willey. 
The pygochord is a structure on which Willey has laid some stress, describing it as 
“a longitudinal, solid, supporting band” (99, p. 243), and evidently seeing in it a skeletal 
structure for the support of the caudal region. On the ground of its minute proportions 
Spengel (O08, p. 317) dissents from Willey’s interpretation, and one cannot help agreeing 
with Spengel in refusing to regard such a small and rudimentary structure as of any 
importance from this point of view. As Spengel remarks we have absolutely no knowledge 
of the function of this apparently vestigial structure, and it is with some diffidence that 
I venture to suggest that it may be the remains of a ventral siphon which was at one 
time of functional importance but now vestigial. The occurrence of such a siphon, whatever 
may be its exact function, is common among sand-feeding animals. Similar structures occur in 
the Echinoidea, in the Capitellidae, and in certain Chaetopoda such as Thallasema and Echiurus 
—all creatures which derive their nutriment from among the large quantities of sand con- 
tinually passing through the alimentary canal. The condition of the pygochord found in a 
specimen of Pt. saxicola is interesting in this connection. The structure in question was 
here connected at either end with the intestine whilst for the middle part of its course it 
was entirely free (cf. Pl. XLIV. figs. 835—37), a condition strongly suggestive of a collateral 
intestine. 
The lateral septum is described by Willey as passing to the epidermis at the base 
of the genital pleurae in the post-branchial region (99, Pl. XXIX. fig. 14). Spengel, 
however, finds that this is not so, but that it ends at the lateral blood vessel (O38, p. 297). 
It is sufficient to remark that on this point I find myself in agreement with Spengel. 
