ANNELIDA 263 



and sperm pass from the coelom to the exterior. In most of the poly- 

 chaets the eggs are fertiHzed externally, forming a trochosphere larva, 

 the method of reproduction thus conforming to that of other marine 

 groups. In the terrestrial and freshwater oligochaets (as in leeches) 

 fertilization is internal and the young hatch in a form resembling the 

 parent. There is no doubt that the former mode of development is 

 more primitive. 



The nephridia are essentially tubes developed from the ectoderm 

 which push their way inwards so that they project into the body 

 cavity. In some polychaets they end blindly — this is the primitive 

 condition. In the majority of chaetopods they have acquired an 

 opening (nephrostome) into the body cavity itself. In some cases there 

 is a partial fusion with a mesodermal element, the coelomoduct, so 

 that a compound tube consisting mainly of ectoderm but partly of 

 mesoderm exists {nephromixium). Nephromixia may take on the 

 functions of coelomoducts where these do not exist independently. 

 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 with venous blood. The whole of the 

 dorsal vessel (Fig. 201) is usually contractile : there may also be vertical 

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

 some forms, for example Pomatoceros (Fig. 186 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. 189)): 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. 



