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ON THE ArLACOrilOROUS AMPHINEURA OF THE BRITISH SEAS. 



By Walter Garstang, M.A., F.Z.S. 



Read 12th June, 1896. 



PLATE X. 



The remarkable worm-like Molluscs now associated with the 

 Chitons in the order Amphineiira, possess a high interest, owing to 

 the unique way in which these animals combine great simplicity of 

 appearance and structure with distinctively Molluscan features. 

 Beneath their simple, woi'm-like guise, however, these creatures 

 betray unmistakable signs of retrogi'ession from a higher grade of 

 organization. The ciliated furrow along the ventral surface of the 

 body looks like a persistent elongated blastopore, or the ciliated 

 ventral surface of an Archiannelid. There can be little doubt, 

 however, that it is a relic of an ancestral mantle-cavity like that 

 of a Chiton. The slender, razor-like fold enclosed within the groove 

 is an heirloom from ancestors with a well-developed creeping foot. 

 The pharynx, smooth in some forms, armed with a single tooth in 

 others, is furnished with a typical radula in certain other types. The 

 coat of fine spicules which envelops the body looks like a primitive 

 form of exoskeleton. Yet Pruvot tells us that in one instance, at any 

 rate, the larva has a series of well-developed Chiton -like plates along 

 its back, which appear to be shed when the creature settles down to 

 its typical mode of life. There seems to be ample reason, therefore, 

 for regarding, with Simroth, the Aplacophorous forms of Amphineura 

 as degraded from a more Chiton-like ancestral condition. Their 

 residence in deep water, beneath the tidal zone and beyond the 

 influence of waves and storms, has enabled these creatures — as com- 

 pared with the littoral Chitonidfe — to dispense with their plate-like 

 armature and suctorial foot, and to adopt new habits and assume new 

 forms. Some burrow in mud ; many, however, lead a semi-parasitic 

 existence, creeping about on the surface of Alcyonarian corals or 

 twining their flexible bodies round the stems of Hydroid Zoophytes. 



In the hope of drawing the renewed attention of English 

 malacologists to this inviting group of Molluscs, I give below a list 

 of the forms which are at present known to inhabit British seas. 

 Two of these have been recorded within the last fifteen months. This 

 fact, when the number of forms now known from neighbouring seas 

 is taken into consideration, renders it very probable that the British 

 list of these Molluscs is far from completed. 



The classification and nomenclature adopted is that of Simroth's 

 recent revision (1893). 



APLACOPHORA. 



I. Fam. Ch^etodeematid^. 



Body constricted into three segments. Ventral furrow absent. 

 Mantle- chamber reduced to a small posterior cloaca. Two gill-plumes 

 in the cloaca. Radula in form of a single tooth. 



Habits. — Burrow in mud. No British representatives. 



