GEPHYREA TNERMIA—BENHAM. 
9 
Fischer (1896, 1914) has recognised that P. capsiforme is but a variety 
or sub-species of the northern P. margaritaceum, and would even place as 
varieties of this species the three South Georgian forms of Michaelsen. 
Lanchester (1908), in his analysis of the form he calls P. socium, seems 
also to suggest the identity of, at any rate, P. fuscum and P. antarcticum, 
and points out that P. socium presents some internal features which resemble 
the one, and others that recall the other of these two forms. 
Tlicel (1911) has gone even further and places all these forms, not as 
varieties, but as members of the northern species. 
Having discussed the characters usually employed in distinguishi lg 
species in this genus, Lanchester concludes that they are of “a very vague 
and unsatisfactory kind,” and relies almost wholly on the character of the 
skin. P. socium agrees more nearly with P. margaritaceum in this feature 
than with the South Georgian forms, in which the papillae are longer. 
Nevertheless, he separates P. socium from P. margaritaceum on account of the 
proportion of introvert to body length; for instead of having an introvert 
about half the length of the body, as in the northern species, it is in the Cape 
Adare form, “ not much shorter than the length of the body.” 
My studies of the specimens obtained by the s.s. ‘'Aurora’ seems 
to me to emphasise the unsatisfactoriness of the usual diagnostic features. 
When we bear in mind the nature of the body wall and the great 
contractility of the whole worm, it certainly appears that we cannot put 
much reliance on proportions of length of the various regions or upon such 
features as the exact position of the origin of the retractor muscles of the 
introvert: or these characters must be used in association with other features. 
The Sipunculids, as is well known, inhabit the mud and sand of the 
sea bottom, and as they burrow therein* must be constantly altering the 
form of the body, pushing forwards the introvert and again retracting it as 
it moves along through the mud. 
When the introvert is fully extended, the circular muscles of the body 
wall as well as those of the introvert must be in a state of contraction, in 
order to force the ccelomic fluid into this introvert and so distend it. But 
the longitudinal muscles may also contract at the same time or immediately 
after, shortening the hinder part of the body, as the animal progresses 
forwards, pulling it after the introvert. Now it is conceivable that the muscles 
of the body and those of the introvert are not contracted to an equal degree 
*4998 —B 
* Andrews, Studies, Johns Hopkins Univ., 1887-1890, vol. iv, p. 389. 
