242 THE INVERTEBRATA 



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 it 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 blind end projecting 

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

 a very close resemblance to the flame cell of Platyhelminthes and 

 Rotifera. Such ''closed" nephridia 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. The familiar example of this is the "nephri- 

 dium" of Lumbricus. In this type the tube mainly consists of 

 ectoderm ; the funnel of the nephrostome is in Lumbricus (and probably 

 other forms) derived from a mesoblast cell. Such an organ pro- 

 duced by the fusion of two different elements is termed a nephromixium. 

 An example which shows very clearly the dual nature of the nephro- 

 mixium is the segmental organ of the leeches. In Fig. i8i it will be 

 seen that the lumen of the proximal (ectodermal) and that of the 

 distal (mesodermal) portions do not communicate and the nephro- 

 stome cannot act as a funnel leading to the exterior. But usually the 

 development of segmental organs has not been sufficiently studied to 

 decide what type they belong to. In the following paragraphs such 

 doubtful cases will be referred to as nephridia. 



The coelomoduct is entirely formed from mesoderm and usually 

 has a wide coelomic funnel easily distinguished from the typical 

 nephrostome. The oviducts and the sperm ducts of Lumbricus are 

 coelomoducts. In a family of the Polychaeta called the Capitellidae 

 there are coelomoducts in most segments of the body serving as 

 gonoducts (Fig. i8o I, D). 



