Characteristics of Annulata. cxxix 



tions, but its absorbing surface is very ordinarily largely increased 

 by the development of lateral sacculations. It is very often armed 

 with a protrusible, muscular and dentigerous, proboscis. Tbe mouth 

 opens ventrally, and the anus dorsally or terminally. A perivisceral 

 cavity is present in all Annulata except certain Disco2)hora, in 

 which it becomes obliterated in the course of the evolution of the 

 internal organs. In this cavity, wheu present, tbe true corpus- 

 culated blood is ordinarily contained. In some Annulata which are 

 called ' anangian/ tbere is no 'pseudhaemaF vascular system deve- 

 loped, and in them the internal surface of the perivisceral cavity is 

 richly ciliated, and the circulation and aeration of the corpusculated 

 fluid it contains thus secured. But in most members of the class 

 we find a closed system of contractile vessels, which, as they contain 

 a fluid which, though ordinarily coloured, is not corpusculated, are 

 called ' pseudhaemal,^ and appear to be respiratory in function ; 

 and iu these worms the perivisceral cavity shows no other ciliation 

 than that of the mouths of the segmental organs which open into 

 it. In a few Anuelids again {SijUldea armata, the Opheliae, the 

 Cirratulida, and the Staiirocepha^i and Branchiobdella), the so- 

 called ' pseudhaemaF system contains corpusculated blood, and com- 

 municates with the perivisceral cavity so as to form a lacunar 

 circulation. Ai'borescent branchiae are not rarely developed in the 

 Polychaeta on the dorsal aspect of the parapodia ; and the tactile 

 cirri ordinarily present on the same, as also on the ventral aspect 

 of the locomotor outgrowth, must possess an aerating as well as a 

 sensory function. The branchiae are often ciliated, and contain 

 afferent and efierent branches of the pseud haemal system, which 

 run parallel and are connected with each other by a double series- of 

 vascular loops within the aerating lamina. Nearly all Annulata 

 possess certain organs, known as the ' segmental organs,"* which 

 open externally on the surface of the bod}', and internally into the 

 perivisceral cavity. The internal orifice is wanting in some Poly- 

 chaeta [Cajntellae) as also in some Discophora ; a part of the often 

 complexly convoluted canal which they consist of is glandular, and 

 as the movement set up by the cilia with which they are lined, 

 ordinarily sets outwards, they may be supposed to be depuratory 

 in function, except in those segments in which they are specialized, 

 as they are in all Annulata except the Biscophora, to act as efierent 

 ducts for the generative products. In some cases, however, as in 



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