80 E. RAY LANKESTER. 



their true or causal relationships, when he said, " Was 

 wiirden wir von einem Architecten sagen, der durch eine 

 Seitenthiire in einen Palast gekommen wave, und nun, hei 

 Beschreibung und Darstelhmg eines solchen Gebaudes, alles 

 auf diese erste untergeordnete seite beziehen wollte? Und 

 doch geschieht dies in den Wissenschaften jeden Tag." 



It is possible that in this way we may be led to look at 

 the Mollusca through a side door, if we do not remember 

 that the simplest forms referable to that group, known to us 

 at this day, are not necessarily the most nearly representa- 

 tive of the Molluscan ancestry. 



In Rhahdojileura we find a large and well-developed foot 

 (the buccal shield), justifying the previous assumption that 

 the epistoma of the fresh-water Polyzoa represents the foot 

 of Mollusca. 



In Brachiopoda there is even less trace of the foot than is 

 afforded by the Polyzoan epistoma ; but in Terehratula there 

 is a bare indication of it — a so-called lower lip. Would it 

 be right from this to conclude that the Brachiopoda are more 

 nearly related to the Vermes, and that we have an ascending 

 series from them, through the Hippocrepian Polyzoa to 

 Rhabdopleura, and thence to Mollusca proper? It seems 

 not ; but far more probably we have a descending series — a 

 loss of this powerful foot — accompanying the acquisition of 

 nnmobility and subsequent arborescent stock-building. 



With the assumption of these habits we have further the 

 abortion of the cephalic region common to Polyzoa, Brachio- 

 poda, and Lamellibranchs J and further, the huge develop- 

 ment of the gill-tentacles — not as respiratory organs, for 

 which function they are far larger than needful — but in all 

 three classes as excitors of currents, by means of their cilia, 

 bringing food to the mouth. The gill-tentacle of Rhabdo- 

 pleura is even more like to an arm of Terehratula than to one 

 side of a hippocrepian lophophore. Still more justly may 

 it be compared in form and in relation to other parts with 

 the gill-plume of some Gasteropods ; whilst its relative, posi- 

 tion as regards foot, mouth and anus, is precisely that of the 

 budding gill-laminae of a young Laniellibranch. 



In the accompanying woodcuts I have diagrammatically 

 indicated these fundamental points of agreement among 

 Acephalous Mollusca. The comparison of the Cephalophora 

 with the Acephalous forms is rendered easy through the 

 Chitons on the one hand and the Lamellibranchs on the 

 other. 



It has not been my object in the above few lines to discuss 

 the details of Molluscan morphology, but to give what I believe 



