NOTES ON EMBRYOLOGY AND CLASSIFICATION. 425 



org-ans. In reality, they are only specially modified parts 

 of the architrochal band, not giving rise to filaments, but by 

 a late modification of an original series of filaments giving 

 rise to spongiose erectile lobes. They complete, anteriorly or 

 prseorally, the architroch of the Lamellibranch. 



The reduction of the filiferous architroch in the Polyzoan 

 E,habdopleura to a plume is of the greatest importance, 

 because it allows us to assume, in other cases, that branchial 

 plumes hate possibly been developed by reduction of an archi- 

 troch. 



Whilst the larvse of certain Echinoderms (the Pluteus and 

 Auricularia of Echinids and Holothurians) are architrochic, 

 the Bipinnaria and Brachiolaria of Asterids present us with 

 an important modification of the primitive condition, as does 

 also the Auricularia of Holothurians, when it passes into the 

 poiytrochous condition. The ring surrounding the mouth 

 becomes drawn out in such a way that it extends round the 

 larva on each side (fig. 4), and its two extremities meeting- 

 and joining we have, as Gegenbaur has shown, two rings 

 developed, whose plane is at right angles to that of the 

 original single peristomial ring from which they develop. 

 The anterior circle embraces the prostomium, the posterior, 

 which is usually larger and oblique in direction, is metas- 

 tomial. We find in the Echinoderms very near approaches 

 made to this breaking of the architroch by a dorsal nip- 

 ping-in, without the actual fusion being accomplished. 

 I propose to call that condition of the ciliary tract in which 

 the fusion is accomplished " zygotrochic.'' The Asterid larvae 

 (fig. 12) are zygotrochic, so it appears is Tornaria (fig. 8), 

 the larva of Balanoglossus. The two secondary circlets 

 into which the architroch divides may be conveniently dis- 

 tinguished as the " cephalotroch "and the "branchiotroch." 

 The "cephalotroch " is also known as the "velum." It is 

 this portion only of the differentiated architroch which 

 makes its appearance in the larvse of the Gasteropodous 

 Mollusca ; it is this part only which appears in the case of 

 many Annelid larvse and in the Rotifera. The " branchio- 

 troch '^ is so named on account of the fact that it is this 

 portion of the differentiated architroch which most con- 

 stantly gives rise to ciliated branchial filaments. Such are 

 • the filaments of Actinotrocha (fig. 13) ; such are the gill- 

 filaments of the Lamellibranchs. I am inclined to think 

 that gill-filaments in the adult, which cannot be directly 

 traced to a larval branchiotroch, in fact, in cases where the 

 larva possesses only a " velum," or cephalotroch, may yet be 



