424) PROFESSOR LANKESTER. 



The telostomiate planula, I conceive, ceased to develop cilia 

 uniformly over its body-surface and acquired a special cir- 

 clet of these appendages, not far distant from the mouth 

 (PI. XXV. fig. 1). The shifting of the main axis and ac- 

 quirement of the prostomiate condition during later develop- 

 ment would bring about the form now exhibited in the early 

 phases of the ontogeny of Echinoderras, with which Gegen- 

 baur starts his consideration of the subject. This form of 

 girdle with the mouth in its centre may be termed the 

 *' Architroch " (figs. 2, 3). I cannot call to mind any 

 existing representative of a telostomiate architrochophor 

 (the cyclostomous Polyzoa are so only by recurrence), but 

 the early stages of Echinoderms are prostomiate or me- 

 taxial architrochophors. So, too, is Actinotrocha, whilst the 

 tentacles of Phoronis are simply an architroch drawn out 

 into filaments. The gill filaments of the Polyzoa, with hip- 

 pocrepian, and circular lophophor, alike correspond to a com- 

 plete architroch drawn out into more numerous processes 

 than we find in the Pluteus-larvse or in Bi[)innari£e, and must 

 not be mistaken in consequence of their filamentary modifi- 

 cation (figs. 17, 19). This development of tentacle-like fila- 

 ments along the line of the ciliated band is a quite common, 

 indeed, characteristic feature of the architroch and the cir- 

 clets into which it divides. Thus in Rotifers (Stephanoceras), 

 and in Gastropod embryos (Macgillivrayia, Ethella), the 

 ' velum ' is drawn out into filamentous ciliated tentacles 

 (fig. 15j. 



The gill-filaments of the Lamellibranchs, together with the 

 labial tentacles, form an incomplete architroch (fig. 16). To 

 be complete the line of origin of the double set of gill-fila- 

 ments which form the gill-plates,^ should be continued on 

 each side behind the foot (between the foot and anus). At 

 this part tlie Lamellibranch's architroch is broken, but this 

 will not appear surprising when the case of Rhabdopleura 

 (see this Journal for 1874) is considered, in which alone, 

 amongst Polyzoa, the architroch is incomplete and reduced 

 to a pair of plume-like appendages ; still less is it to be 

 wondered at that the architroch should be broken through 

 at this point, when we consider the enormous development 

 of the muscular lobe within the architrochal area, the foot, 

 truly a hypertrophied chin. 



An important consequence of the view now advanced as 

 to the nature of the gill-filaments of acephalous MoUusca is 

 the serial homology of the labial-tentacles of Lamellibranchs 

 with the gill-plates of the same animals. Continually all 

 sorts of special homologies have been proposed for these 

 ' See Holman Peck, this Journal, January, 1877. 



