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, lie 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 I.acaze-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 Dondeysia 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 MoUusca 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 MoUuscan 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. 



