Xl] MAIN DIVISIONS 169 



true of the Gephyrea tubicola and there are a great many other 

 features in which the Gephyrea tubicola and the Sipunculoidea 

 resemble one another. We may therefore with great plausibility 

 regard these two groups as forming an independent group of animals 

 or phylum for which the name Gephyrea may be reserved, the 

 Echiuroidea being removed to the Polychaeta and the Priapuloidea 

 being left on one side for the present as a completely isolated group 

 until we know more about their development. 



The Priapuloidea differ not only from the Sipunculoidea but 

 from all the groups classed together as Gephyrea in the character 

 of their excretory and genital organs. The genital organs are in the 

 form of sacs, from the walls of which the ova and spermatozoa are 

 produced, and which are directly continuous with the genital ducts, 

 of which there are a pair opening to the right and left of the anus. 

 When young specimens are examined no genital organs or genital 

 ducts are to be found, but in their place two typical nephridia 

 which end internally in a number of branches, each branch terminat- 

 ing in a flame-cell or solenocyte. As the animal grows, pouches are 

 given off from the nephridial duct, the blind ends of which 

 develop into the genital organs whilst the nephridial duct 

 becomes the genital duct. Numerous specimens of the genus 

 Priapulus, which has two branched gills protruding from the hinder 

 part of the body, are found embedded in black mud in moderate 

 depths in the Firth of Clyde. 



The Gephyrea tubicola consist of a number of species which are 

 grouped in a single genus PJwronis. The British species are minute 

 creatures about of an inch long. The mouth is surrounded by 

 hollow ciliated tentacles which spring from a lip or platform termed 

 a lophophore, and this lophophore is not circular but drawn out 

 at the side into two arms which project dorsally and give the 

 whole the form of a horse-shoe. These tentacles are supplied with 

 branches from dorsal and ventral blood-vessels and serve for 

 respiration as well as to bring food. The anus is situated on the 

 dorsal surface far forward and two ciliated funnels which serve both 

 as genital ducts and excretory organs open at the sides of it. When 

 Phwonis is young it swims about in the sea by means of an oblique 

 ciliated band running behind the mouth and much resembles the 

 larva of some Annelids. There is a large prostomium in the form 

 of a hood overhanging the mouth and the anus is at the posterior 

 end of the body. As the larva grows an intucking of the skin 

 on the ventral surface makes its appearance. This becomes attached 



