THE ESSEX FIELD CLUB.
47
associated parts, as something at the time of its discovery wholly new, Prof.
Howes proceeded to show it to be identical with that shortly afterwards
discovered in the developing Yoldia by Drew; and he then showed the
detailed differences between the two to be comparatively insignificant, and
probably due to adaptation.
Commenting upon the facts, he regarded them as of supreme importance,
when considered together with those of adult anatomy; and he expressed the
belief that they involve the Acephala and the Polyplacophora in a direct
genetic relationship—a community of origin from some bilaterally symmetrical
ancestor. He argued that the facts altogether outweigh the supposed
Rhipidoglossan affinity of the Acephala with the Gastropoda, which as a
group are asymmetrical.
He next passed to the work of Lacaze-Duthiers, Kowaleyski, and others,
on the structure and development of the Scaphopoda (Dentalium), a group of
Molluscs of world-wide distribution, whose affinities have always been much
in dispute. Concerning their development, he agreed with Drew that their
4—5 rowed ciliary girdle, which, at first equatorial, by forward translocation
and reduction of parts becomes a complex velum, is probably the homologue
of the test of Dondersia and Yoldia ; and he said that if this be so, Dentalium
might hark back with these to a common ancestrial series.
Prof. Howes next sought to show it to be a conclusion from these data
that the velum is either a concentrated and translocated test, or the test an
overgrown velum ; and he also sought to prove that the existence of the test
explains such hitherto anomalous features as the duplication of the trochal-
ridge in the trochosphere of Patella and the presence of a pre- and post-oral
ciliated annulus in the larva of the Ship-worm (Teredo), if not the polytrochal
stage generally and its variants.
In conclusion Prof. Howes dealt with the habits of Dentalium, and
indicated the structural features which that animal presents in common with
the Acephalous and Cephalous Mollusca respectively, and considered more
especially its shell, which when fully formed is a tapering tube. He showed
that, like this, the "shell" of the Acephalous Mollusca (dealing with the
"Gaper" and certain boring species) really forms, with its associated parts, a
complete tubular investment, and that its specially characteristic feature—the
calcification of its opposite halves—is said by the late eminent conchologist,
Paul Fischer, to be passed through during development by Dentalium itself.
He emphasised the need of confirmation of this remarkable statement, and
deduced the final conclusion that the points of morphological community
between the three Molluscan groups which had been considered, simplified
our conception of their inter-relationships to a degree as unexpected as it was
satisfactory. By these discoveries a really great advance had been made. It
strengthened our confidence in the larval form as a guide to affinity, and had
come as a welcome sequel to recent work in America upon the Trilobites
which had similarly justified our trust in the Nauplius.
Prof. Howes wound up by an earnest appeal to the members of the Essex
Field Club, to seek, with all possible speed, a further knowledge of the
developmental history of their indigenous marine Acephala.