INSECTA 129 



Mollusca and the Annelids, in endoproct Bryozoa, and also, 

 as we shall find, in the vertebrate Amphioxus. They are 

 developed as temporary organs of excretion in many Mollusca, 

 and in land and aquatic Annelida. They are also formed in 

 larval Sipunculidea, Brachiopoda, and Phoronidea, and in 

 these they are converted by forming an opening into the coelom 

 into an organ which, continuing its function of excretion, 

 assumes that of a gonoduct. The growth of the Annelid and 

 its characteristic meristic segmentation have led to a series 

 of protonephridia being formed. The anterior ones are sup- 

 pressed that is, after the embryonic or larval phase, during 

 which they are associated with the primary body cavity, or 

 head cavity, as it is sometimes called. The posterior ones, 

 developed in succession in regular order from the ectoderm, 

 are put into communication with the coelom and are nephridia. 

 In some cases they may act as gonoducts. 



The larval nephridia of Mollusca are altogether absorbed, 

 and the coelom develops another kind of duct, as an outgrowth 

 from the coelom that is, it has a mesodermal origin and gains 

 an opening through the body wall. It recalls, and may be 

 said to be homologous to, the terminal ducts of the gonocoel 

 of the Platyhelmia. The coelomiduct acts as a gonoduct in 

 Annelida, and in the Polychaeta in certain cases it may be 

 conjoined to a nephridium. 



The coelom of the Arthropods undergoes great reduction. 

 It may be that the tough cuticle of the Arthropods has made it 

 unnecessary to have a watery distension of the body, such as 

 the coelom provides in their softer-skinned allies. The result 

 has been that no nephridia are formed, and such ducts as are 

 produced in relation to the gonads and the kidneys of 

 Crustacea are coelomiducts. 



The Metazoa illustrated by the preceding examples are 

 characterised by a fundamental feature of development. The 

 blastopore is carried in to form the mouth, or, if it be closed, 

 it marks the place where the mouth will arise. For this reason 

 this section of the Metazoa could be linked together as 

 Stomoporida. The polarity is seen in the Coelenterata, where 

 the anterior end is the fixed end, and the posterior end that 

 from which the tentacles are produced and at which the mouth 



