NOTES ON EMBRYOLOGY AND CLASSIFICATION. 427 



The most remarkable polyepitrochic larvne are those to 

 which the Auricularian architrochic larvae of the Holothu- 

 rians give rise. Tlie architroch of these larvae becomes a 

 zygotroch, a distinct cephalotroch or velum, belonging to 

 the prostomial region, being pinched off from the branchial 

 circlet ; but this latter, instead of remaining as a single 

 girdle, again breaks up into four circlets by development of 

 cross-pieces in correspondence with a metamerism, often 

 indicated also by the deeply-marked lobulation of the body. 

 It is interesting to note that in this transient metameric 

 segmentation of the Hololhurian, the metameres agree with 

 the more fully developed metameres of lineally segmented 

 worms in the fact that, ichilst the first metamere consists of 

 iwostomium and metastomium, each succeeding metamere cor- 

 responds only to the metastomial portion of the first metamere. 

 This is clear in the case of the polyepitrochic Holothurian 

 larva, since the metastomial circlets are developed by direct 

 conversion of the metastomial portion (branchiotroch) of the 

 zygotroch. In other polyepitrochic larvae, as well as in 

 monepitrochic larvae, the epitrochs develop quite indepen- 

 dently of the architroch or its parts. Terebratula and other 

 Brachiopods present us with a polyepitrochic larva havino' 

 an architroch anteriorly. Actinotrocha has one epitroch and 

 an architroch. The larva of Dentalium possesses a cephalo- 

 troch, followed by many epitrochs independently developed. 

 The same is true of some Pteropod larvae. Many Chjetopod 

 larvae are also in this condition. 



It seems to be quite possible that the branchial filaments 

 of Chaetopod worms, as well as the branchiae of the Eucepha- 

 lous Mollusca, may, like the branchial filaments and plumes 

 of the Acephalous Mollusca and the tentacular crowns of 

 Phoronis and the Gephyraea, be ultimately traceable to the 

 branchiotrochal moiety of the architroch. 



Enumeration of the modifications of the Architroch. 



A. — Architrochic forms : 



1. Anepitrochic — 



Pluteus of Echinids and Ophiurids; Auricularia of 

 Holothurian ; larval and adult Polyzoa ; Lamellihranchs 

 and Brachiopods; adult Phoronis; and also Bonellia, 

 Thalassema, Sipunculus. 



2. Monepitrochic- — 

 Actinotrocha. 



3. Polyepitrochic — 



Larval Brachiopods (Kowalewsky). 

 B. — Zygotrochic, with separated cephalotroch and bran- 

 chiotroch : 



