206 EDMUND B, WILSON. 
with rudinients of the contractile czeca (c.), which are so 
striking a peculiarity of the adult circulatory apparatus. 
Figure 8 represents a considerably later stage, in which is 
shown a great advance in the development of the pouch. 
Bending at first toward one side of the body it folds upon 
itself, and returns to the opposite side, then turns sharply 
downward along the intestine, and finally bends upward 
again near its extremity. Its inner walls are transversely 
folded. ‘The arms have increased to the number of twenty, 
and are thickened on the lower side just at the base, to form 
the first rudiments of the permanent tentacles. There are 
four masses of pseud-hemal corpuscles, two lying at each 
side of the stomach. 
Figure 9 gives a dorsal view of the same stage, to show 
the arrangement of the arms on the dorsal side. The folds 
of the pouch have here a somewhat different arrangement, 
and cannot be clearly followed. The rudiments of the ten- 
tacles are shown at p.a., and the broad contractile dorsal 
vessel at v. 
Figure 10 represents a full-grown larva of B immediately 
before its metamorphosis. The folds of the pouch are 
voluminous, and occupy a large part of the perivisceral 
cavity ; they have nearly the same disposition as in fig. 8. 
The tentacular rudiments are very distinct, and have become 
partly independent of the larval arms. Both dorsal and 
ventral pseud-hzmal vessels are well developed and, like the 
ceecal appendages, are actively contractile. The masses of 
corpuscles (cor.) are large and very conspicuous from their 
red colour. Form A agrees essentially with B at this stage, 
though the body is much stouter, and the rudiments of the 
tentacles are much smaller and more closely united to the 
larval arms. 
The two species differ slightly as to the manner in which 
the metamorphosis is effected. In the case of A the larva 
sinks to the bottom, contracts very strongly, and the evagi- 
nation of the pouch occupies but a few minutes. The pro- 
cess occupies in the case of B a much longer time, and the 
body is only slightly or, at first, not at all contracted. 
Figure 11 represents the latter species at the erdtical 
stage, with the pouch about half unrolled. The inner ex- . 
tremity of the latter still extends to the back of the stomach. 
At 6,6, are seen some of the fibrous connections between the 
intestinal walls and those of the pouch. ; 
Figure 12 represents the creature with the pouch (the 
whole of which is not shown) wholly unrolled. The hood 
has been withdrawn into the cesophagus, the larval arms 
