NERILLA AN ARCHI ANNELID. 
421 
open funnels, a slightly coiled lumen, and vacuoles con- 
taining one refringent concretion. Nephridia are present in 
segments 2, 5 and 9 of the male, and in segments 2, 5, 6 
and 8 of the female. The male has three genital segments, 
5, 6, and 7. In the first only do the testes produce sperma- 
tozoa ; in the second and third the testes appear to he 
degenerate and to produce granular cells, with small granules 
in segment 6, and large granules in segment 7. Three pairs of 
similar ducts correspond to these three segments. The six 
sperm-ducts open into a common median ventral genital atrium, 
which leads to a median pore with a small copul atory process on 
each side. Special epidermal gland-cells open into the genital 
atrium. Two ovaries are present in the sixth segment of the 
female. Each ovary becomes enclosed in an ovisac, lined by 
nutritive epithelium ; the ovisacs are asymmetrically deve- 
loped. Full-grown ova escape from the sacs into the coelom 
of the sixth segment, and are retained for a long time in the 
parent, passing forwards or more often backwards along each 
side of the gut. Here they undergo the preliminary stages 
of maturation. It is possible, but not yet certain, that fertili- 
sation is internal. A pocket, containing deeply staining cells, 
is found on the inner side of each ovisac. Two oviducts are 
present, leading from the sixth segment to paired genital 
pores on the seventh segment. Epidermal glands converge 
towards the pores. The eggs are laid in transparent capsules, 
development is direct, and the young emerge as small worms 
wit.ii only one median tentacle and about five segments. 
Nerilla combines primitive with specialised characters. 
The parapodia, cliaetae, coelom, and number of the gonads and 
their ducts have become progressively reduced in the Arclii- 
annelids. They are probably modified forms descended from 
Chaetopod ancestors. The genital ducts appear to lie either 
coelomostomes, or ccelomostomes combined with nephridia 
(nephronixia). Nerilla preserves many Chmtopod characters, 
but has affinities with several genera, more especially 
perhaps with Histriobdella and Dinophilus. 
