x PHYLUM ANNULATA 461 



posterior nephridia. A rudimentary prse-oral lobe becomes esta- 

 blished. The mesoderm remains unsegmented, but splits into 

 somatic and splanchnic layers going to form the muscular system, 

 blood-vessels, and other mesodermal organs. Before the alimentary 

 canal is formed the larva, which had previously been spherical 

 with two bands of cilia and a pair of eye-spots, becomes elongated 

 and dorso-ventrally compressed ; and becomes covered uniformly 

 with cilia so as to present the general appearance of a Planarian. 

 It becomes converted into the adult female by a metamorphosis, 

 including the elongation of the prse-oral lobe to form the proboscis, 

 .and the development of the pair of setas of the adult. The 

 male never goes through this metamorphosis, but remains in the 

 Planarian stage : it at first adheres to the proboscis of a female, 

 then enters the oesophagus, and afterwards, when sexually mature, 

 enters the cavity of the nephridium. 



In the Inermia the early stages of the development closely 

 resemble those of the embryo of one of the Polychaata, and a 

 trochosphere is formed, closly resembling the corresponding stage 

 in the latter class. But at no stage in the development has any 

 trace been observed of the temporary segmentation which forms so 

 marked a feature in the development of Echiurus. 



Distribution, Affinities, etc. The Gephyrea are all marine. 

 They are only capable of very slow creeping locomotion, and live 

 for the most part either in natural rock-fissures, or in burrows 

 which they excavate for themselves either in sand or mud, coral or 

 rock. Their distribution is general ; and they occur at considerable 

 depths as well as in shallow water. 



The differences between the Inermia and the Armata are so 

 considerable, that there is some doubt whether they ought to be 

 united together in one class. The Inermia diverge most widely 

 from the Chastopoda in the entire absence of setae and in the 

 .absence of segmentation at any stage. 



Affinities between Phoronis (p. 328) and the unarmed Gephyrea 

 have often been supposed to exist, and it has by some zoologists been 

 proposed to regard Phoronis as an outlying member of that class. 

 It seems probable, however, that the very manifest resemblances 

 which undoubtedly exist do not indicate a near relationship, but 

 are the result- of converging modifications of originally widely 

 different stocks. The most striking of these points of resemblance 

 are two (l)the approximation of the anus towards the oral aperture, 

 and (2) the presence of the tentacular circlet. But a study of the 

 development shows that these common features are developed in 

 totally different ways in the two cases. The forward position of 

 the anus in the Sipunculida is brought about by a gradual displace- 

 ment resulting from the growtli of the aboral region of the body ; 

 and the invagination and evagination by which the corresponding 

 result is attained in Phoronis do not occur. Again, while in Phoronis 



