EXTEKKAX BEAXCHLE OF VEEMES. 135 



formed by the metamorphosis of their dorsal cirri ; they are scale- 

 like lamella^ which lie on one another on the dorsal surface, and 

 alternate with short processes (Aphroditidae). Although the 

 parapodia of the Annelida, which function as locomotor organs, 

 appear to be the rudiments of the appendages, which are more per- 

 fectly developed in the Arthropoda, they are not independent, for 

 they have no special muscular apparatus of their own, like the 

 appendages of the Arthropoda, and they are principally set in action 

 by the general movement of the metameres to which they belong. 



External Branchiae. 

 § 106. 



The appendages on the head, as well as those on the metameres, 

 of the Chastopoda undergo various changes in adaptation to the 

 respiratory function. Although in most Vermes this is per- 

 formed by the general surface of the body, in the Chretopoda it is 

 confined to definite parts, which are thereby converted into 

 branchiae, as may be seen from their relation to the vascular system, 

 and from other points in their structure. 



The cephalic tentacles are the first to enter into these 

 respiratory relations (§ 104). In some (Pectinaria, Terebella) these 

 structures contain a perienteric fluid, and are not distinctly 

 branchiae. They are more definitely branchial in the Pheruseida3 

 (Siphonostoma). In the Sabellidas they are still further differen- 

 tiated in the manner described above, and the separate gill-filaments 

 are beset with secondary pinnules, by which their surface is further 

 increased in size. 



Just as gills are formed by the special development of the 

 cephalic tentacles, so, too, gills are formed from the appendages 

 of the separate segments of the body, by the modifica- 

 tions of the cirri which are attached to the parapodia, or by the 

 formation of special appendages. When simplest the cirri are not 

 altered in character, but enclose a continuation of the ccelom, so 

 that the perienteric fluid can enter into them. The presence of 

 cilia on the cirri is also of importance for their respiratory 

 functions. The exchange of gases is promoted by the walls of the 

 cirri becoming considerably thinner at certain points. As a rule 

 it is the dorsal appendages which are developed in this way. The 

 so-called elytra of the Aphroditidae also belong to this series of 

 processes. They communicate freely with the ccelom. Cirri become 

 more definitely related to the respiratory function by the continuation 

 of the blood-vascular system into them. They then form branchiae. 

 These either retain the condition of simple processes — sometimes they 

 are lamellar in form — or are variously branched. In Cirratulus 

 they are greatly elongated single filaments. The branched form 

 includes the more delicate branchiae, which are either comb-like 



