484 ZOOLOGY SECT! 



vesicles which appear at its sides, developed as outgrowths from the 

 terminal part of the intestine, form the rudiments of the posterior 

 nephridia. A rudimentary pre-oral lobe becomes established. 

 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 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 setae 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, passes into the 

 cavity of the nephridium. 



CLASS II.-SIPUNCULOIDEA, 



The Sipunculoidea are marine Annulata devoid of any trace of 

 segmentation in the adult condition, without parapodia, and 

 without setae ; with an invaginable anterior body-region or introvert, 

 at the extremity of which is the mouth surrounded by tentacles. 

 The anus is anterior and dorsal. There is an extensive ccelome 

 filled with a corpusculated fluid, and not divided by septa. The 

 ventral nerve-cord is not made up of a series of ganglia. There is, 

 as a general rule, only a single pair of nephridia. The sexes are 

 separate ; the ovaries and testes simple masses of cells ; the 

 nephridia act as reproductive ducts. The larva is a modified 

 trochophore. 



1. EXAMPLE OF THE CLASS Sipunculus nudus. 



General External Features. Sipunculus occurs on sand at 

 moderate depths off the coast in most countries outside of the 

 tropics. It is an elongated worm of a cylindrical shape, somewhat 

 narrower towards one the anterior end. There is no trace of 

 division into segments. The anterior portion of the body, to the 

 extent of about a sixth of the total length, is capable of being 

 involuted within the part behind. The surface of this anterior 

 part, which is termed the introvert (Fig. 401), differs in appearance 

 from that of the rest of the body in being covered more or less 

 closely with chitinous papillae. The papillae of the posterior 

 portion of the introvert are shaped like the bowl of a spoon, with 

 the concavity turned towards the body-wall and the tip directed 

 backwards ; they are so closely arranged as to overlap one another 

 like the shingles of the roof of a house : further back they become 



