230 THE INVERTEBRATA 



mixia may take on the functions of coelomoducts where these do 

 not exist. All types of tubes are termed here segmental organs. 



The head and accompanying sense organs may be well developed, 

 for instance, in some of the pelagic Polychaeta where the eyes are re- 

 markably complex. In such cases the brain (prostomial ganglia) may 

 attain a structure almost as complicated as in the higher arthropods. 

 The head processes (tentacles, palps) vary greatly. While they may 

 be very complicated in the Polychaeta, they are frequently absent in 

 burrowing members of that group and invariably so in the Oligo- 

 chaeta. 



The blood system also varies greatly. In small forms it is absent 

 altogether. Typically it consists of a dorsal vessel in which the blood 

 moves forward, and a ventral vessel in which it moves backward and 

 from which the skin is supplied into venous blood. The whole of the 

 dorsal vessel (Fig. 190) is usually contractile : there may also be vertical 

 segmental contractile vessels which are usually called "hearts". In 

 some forms, for example, Pomatoceros (Fig. 175 C), there are no 

 separate dorsal and ventral vessels but a sinus round the gut : the peri- 

 stalsis of the latter brings about the movements of the blood. While 

 the whole of the skin is sometimes richly supplied with blood vessels 

 and usually performs an important part in the aeration of the blood 

 there are often branched segmented processes which may rightly be 

 called gills (Arenicola (Fig. 178)): the alimentary canal is probably 

 a respiratory organ too. While haemoglobin is often present in the 

 blood, usually in solution, a related pigment, chlorocruorin, which is 

 green, occurs in many tubicolous polychaets. The variable state of the 

 mechanism of respiration is shown by the fact that one species of 

 a genus (the polychaet. Poly cirrus) may possess haemoglobin while 

 another has no respiratory pigment. 



The Chaetopoda are, in this work, divided into the following 

 orders: (i) Polychaeta, (ii) Oligochaeta. 



Order POLYCHAETA 



Marine Chaetopoda with numerous chaetae arising from special 

 prominences of the body wall called parapodia ; usually with a distinct 

 head which bears a number of appendages ; nearly always dioecious, 

 with gonads extending throughout the body and external fertiliza- 

 tion; with a free-swimming larva, the trochosphere . 



The structure of the Polychaeta is very variable and dependent on 

 the habit of life, both externally (especially the head appendages and 

 parapodia) and internally (especially the segmental organs). The 

 variation in methods of reproduction is also very characteristic. For 

 these reasons an account will first be given of some of the very large 



