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Proceedings of the Royal Society of Edinburgh. [Sess. 
one, those of Arenicola are invariably situated on the chsetigerous annulus. 
The nephridia of B. vincenti are reduced to two on each side, the second 
of which is elongate and extends through three or four segments ; whereas 
the species of Arenicola cited have five pairs of nephridia, each of which 
extends through one segment only. Other striking differences are exhibited 
by the reproductive organs, which in Branchiomaldane are hermaphrodite 
and extensively distributed on the oblique muscles and septa; whereas 
Arenicola is invariably dioecious and its gonads are restricted to the 
gonadial vessels on the nephridia. The egg of Branchiomaldane is large 
and plentifully yolked and does not give rise to a ciliated larva, while 
the egg of all the species of Arenicola is much smaller, and, in those 
species which have been investigated, produces a free-swimming trochor 
sphere larva. As the relationship of Branchiomaldane is especially with 
the ecaudate species of Arenicola, it is worthy of note that the latter 
possess statocysts, giant nerve-fibres, septal pouches, and a nuchal invagina- 
tion, whereas all these are wanting in Branchiomaldane. But the im- 
portant difference in regard to the position of the gills is alone sufficient, 
in the writer’s opinion, to render necessary the maintenance of the two 
genera Arenicola and Branchiomaldane. 
While Branchiomaldane presents some primitive characters, for in- 
stance, a simple conical prostomium and homonomy of its segments, it 
affords considerable evidence of having undergone secondary modification 
and retrogression. Its small size, the simple form of its gills, the absence 
or great reduction of certain sense-organs (statocysts and nuchal organ), 
the reduction in the number of nephridia, its hermaphroditism, the large 
size of the egg and the absence of a free-swimming larval stage — features 
in which Branchiomaldane departs from Arenicola — are probably correlated, 
to a large extent, with the much more sedentary life of the former. The 
reduction of sense-organs, and the reduction or modification of nejDhridia, 
are well-known secondary changes associated with the abandonment of 
errant for sessile habits. In particular, the occurrence of hermaphroditism 
affords strong evidence of departure from the primitive condition ; for 
hermaphroditism is secondary in Polychseta, as it is in Mollusca. This 
seems clear for, at least, two reasons : (1) because of the few cases of 
hermaphroditism — only about a score of species — known in the Polychseta, 
and (2) because hermaphroditism is generally associated, in members of this 
order, with some obviously secondary modification of structure or mode of 
life. About half the known hermaphrodite Polychseta are tube-dwelling 
Sabelliformia, while most of the others are Polychsetes of unusually small 
size and simplified structure — e.g., Lycastis quadraticeps, Ophryotrocha 
