POLYCHAETA 275 



haemoglobin in solution in the plasma, differs slightly from that of 

 Lumbricus : there is a single pair of large hearts, each divided into a 

 ventricle and auricle which connect the important lateral intestinal 

 vessels from which the branches supplying the gills are derived with 

 the ventral vessel. 



The circulation for that region just behind the heart may be ex- 

 pressed as follows : lateral vessels->auricle^ventricle-> ventral vessel 

 -^afferent vessel to body wall and gill->efferent vessel to subintestinal 

 vessel->intestinal plexus->dorsal vessel or lateral vessel. The dorsal 

 vessel does not communicate directly with the heart. 



The segmental organs are, like the gills, only found in the middle 

 region. They are prominent organs lying beneath the oblique muscles, 

 remarkable for the large size of the nephrostome, the dark secretory 

 bag-like portion, the cells of which contain insoluble excreta, and 

 the small gonad which lies just behind it. In Arenicola as in Lum- 

 bricus the gonads are restricted to a small number of segments, but 

 the reproductive cells are shed into the body cavity at maturity and 

 completely fill it. 



In Glycera the prostomium is narrow and conical, the tentacles 

 being very small. It possesses a very large proboscis armed with four 

 sharp teeth. The parapodia are reduced in size, and bear compound 

 chaetae and in its internal structure too Glycera comes nearer to the 

 errant worms than does Arenicola. 



The excretory and reproductive organs of the Polychaeta 



Now that a survey of the chief types of the Polychaeta has been 

 made a brief description of the segmental organs found in the group 

 will be given. These are tubes, repeated in successive segments, 

 which serve to convey the excretory and generative products from the 

 coelom to the exterior. They are primarily divided into nephridia^ 

 derived from ectoderm, and coelomoducts , formed from mesoderm. 

 The typical nephridium is a closed tube, whose bhnd end projecting 

 into the coelom is fringed with solenocytes, cellular organs which have 

 a very close resemblance to the flame cell of Platyhelminthes and 

 Rotifera. Such "closed" nephridia (protonephridia) are found in 

 the Phyllodocidae, Glyceridae and Alciopidae. But in the majority 

 of the Polychaeta and all Oligochaeta there is another type of 

 "open" tube, which usually serves for the escape of excreta, and this 

 possesses a small funnel or nephrostome. It may, however, take over 

 the function of the coelomoduct and carry sperm or eggs to the 

 exterior. In this case the nephrostome becomes wider and the tube 

 more glandular. The familiar example of the open tube is the 

 nephridium of Lumbricus, which is purely excretory. In this type 



