502 ZOOLOGY 



SECT. 



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 pre- 

 oral lobe to form the proboscis and the development of the pair of 

 setse of the adult. The male never goes through this metamor- 

 phosis, 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, passes into the cavity of the nephridium. 



In the Inermia the early stages of the development closely 

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

 stage corresponding to the trochophore of that class is developed, 

 but with the mouth situated further forward in front of the ring 

 of cilia and with the anus in front of the posterior extremity 

 on the dorsal side. 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 Chaetopoda in the entire absence of setae and in the 

 want of segmentation at any stage. Priapulus differs in such 

 important points from the rest of the Sipunculoidea that 

 it is sometimes regarded as constituting a distinct order. 



Affinities between Phoronis (p. 355) and the unarmed Gephyrea 

 have often been supposed to exist, and by some zoologists it has 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 — (1) 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 arise 

 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 growth of the aboral region of the body ; 

 and the invagination and evagination by which the corresponding 



