457 
intestinal cells. If we admit the truth of any of the speculations‘ re- 
garding the ancestral history of the annelids, it is perfectly easy to 
see that with such a change of environment as they all require a 
strong cuticle would be an essential condition of existence. Hence the 
simple cells would become modified for secretory purposes, and the 
breaking up and distribution of the nucleus would doubtless accom- 
pany this change of function. Hence we arrive at the hypodermis 
stage. Now it of necessity follows that when the transition from either 
a coelenterate- or a planarian-like form to a terricolous annelid was 
being effected, the reproductive organs would become considerably 
modified. Certain it is that accessory copulatory apparatus would be 
necessary, and thus we have the origin of the clitellum. For this 
apparatus, in the process of evolution, would at first consist of 
thickened cuticle secreted by enlarged hypodermic cells, that is, by 
calceo-cuticular glands. Then, a strong connection of course being 
necessary, a further development took place — some calceo-cuticular 
glands being retained to provide cuticular mather, whilst others were 
differentiated for a purely calciferous function (the calceo-cutaneous 
glands), i. e. for providing a stout sheath by the deposition of carbo- 
nates in the cuticular substance. If the clitelline glands were to be- 
come functionless, then both the calceo-cuticular and the calceo-cuta- 
neous layers would atrophy back to hypodermic cells, the former first. 
Hence the presence of two series of glands is rendered intelligible, 
and by a hypothesis which the ontogeny of the clitellum entirely sup- 
ports, or, I should rather say, suggests. A few verbal alterations will 
adapt it to any of the explanations of the function of this structure, 
which differ more in degree than in kind. 
I wish, in conclusion, to express my indebtedness to Professor 
Burdon-Sanderson, F.R.S., who most kindly placed the resources 
of his laboratory at my disposal. 
4 Lang, »Lehrbuch der vergleichenden Anatomie«, Jena, 1888, and »Der Bau 
von Gunda segmentata«, Mitth. Zool. Stat. Neapel, III. 1882, believes the Chaetopods 
to have been derived from a leech-like type (according to Wilson — Journ. Morph. 
Vol. I. No. 1. p. 190, 1887 — this is supported by his observations on the develop- 
ment of the germ-bands of Zumbrieus), which in turn took its origin from a Poly- 
clade Turbellarian, and this again from a Ctenophore—hence indirectly connecting 
the Annelids with the Coelenterates. Balfour (Comparative Embryology«) and 
Sedgwick Origin of Metameric Segmentation«. Quart. Journ. Microse. Se. XXIV. 
1884) believe in coupling the annelids directly with the Coelenterates (the latter with 
the Actinozoa), and Wilson (Journ. Morph. Vol. III. No. 3. p. 441. 1889) states 
that the results of his researches on the trochosphere and the teloblasts of the earth- 
worm might be interpreted either way. Of course, as far as we are concerned, these 
hypotheses may be considered as identical. 
