8 
THE VOYAGE OF H.M.S. CHALLENGER. 
containing prolongations of the intestinal cseca and generative glands. There exist, 
however, a number of transitional forms, which prove that these caudal appendages are 
merely peculiarly modified cirri ; in several species ( Myzostoma jilicauda, Myzostoma 
jiliferum, Myzostoma quadrijilum, Myzostoma intermedium ) the caudal appendages are 
only hollowed out at the base, while the apical portion closely resembles a cirrus. 
In the last-mentioned species (PI. IV. fig. 2) a transition is seen between the cirri and 
the caudal appendages, the outermost pair of which resemble the cirri far more closely 
than the inner pair. In Myzostoma brachiatum (Genus Myzostoma pi. ii. fig. 2) the cirri 
resemble the caudal appendages in being situated ventrally ; the intestinal cseca, 
moreover, penetrate the bases of the larger cirri for a short distance, although in other 
respects they agree in structure with the small cirri ; if the branches of intestine were 
to occupy the whole of the cirrus, it would become exactly similar to a caudal appendage. 
There is little doubt, therefore, that the two structures are homologous, and that both 
have the same conditions of growth. In consequence of which it may be that the size 
of the caudal appendages, as well as their presence or absence, and the length of the 
terminal threads, is only a sign of difference of age — as far as these conditions are 
proportionate to the size of the animal examined. But also the number of the caudal 
appendages is of doubtful value as a diagnostic character if we consider more closely 
the number of the true cirri. 
By investigating individuals of various ages and species, and comparing all the forms 
that bear cirri, it is clear that with regard to the number present there are primarily 
two groups to be distinguished: — (l), those that possess from the very first ten pairs 
of these organs which do not subsequently increase in number; and (2), a second group 
in which their growth is unlimited, new lateral cirri appearing between the ten original 
ones. I am not able to state with certainty, from my examination of the material 
at my disposal, whether the growth of lateral cirri is at all limited, and whether 
there is any order or regularity of sequence in the appearance of the new ones. In any 
case there may be at length species in which the margin of the body is so covered 
that there is absolutely no room for any more (PI. X. of this Report, and Genus Myzos- 
toma, pi. x. fig. 1). There are but rarely less than ten pairs of cirri, if these organs 
be present at all ; the encysted Myzostoma tenuispinum (PI. XIII.), and Myzostoma 
willemoesii (PI. XIV.), however, have only seven pairs. Besides the cirri there are in 
one species — Myzostoma jimbriatum (PI. VI. figs. 5, 6) — bunches of fine threads along 
the margin of the body, which probably serve as tactile hairs. 
Parapodia. 
The parapodia have been already discussed, in so far as they influence the symmetry 
of the body. In some of the new species there is a new form of parapodium, in which the 
terminal portion is not a tube from the extremity of which protrude the hooks, but has 
