420 Causes and Course of Organic Evolution 



vessels, and of segmental organs, suggest embryonic somites. 



Lateral sensory lines are scarcely traceable in rotifers or in 

 tiirbellarians, since the ciliated disk of the former and the 

 ciliate surface of the latter are highly sensory. But the so- 

 called "marginal groove" of some turbellarians may yet be 

 homologized with that now to be noted. 



In nemerteans and all higher forms such as the cyclos tomes 

 and urodeles two highly sensitive lateral lines are connected 

 with the lateral nerves, and only in mammals — with their 

 hair covering — is the sensitive relation and the recognized 

 morphological aspect of it largely reduced. But even in man 

 lat:eral tracts are more sensitive than are the dorsiventral 

 regions. 



The circumoral area in all is circular to elliptic and is sur- 

 rounded by swollen ridges or lips except in simpler rotifers 

 and in turbellarians. In the latter groups the mouth leads 

 into a buccal or pharyngeal region that is chitinous and pro- 

 trusible as a short proboscis, and which can be retracted into 

 the top of the gullet or oesophagus. This pharynx develops 

 rods, teeth, or hooks of a horny nature that are used when the 

 proboscis is protruded. In the Nemertinea the proboscis may 

 vary from a short to a greatly elongated highly sensitive body 

 that can be rapidly shot out or retracted again into a special 

 cavity — the proboscis sheath or rhynchocoel. iVs this struc- 

 ture assumes special importance in connection T\^th the phy- 

 logeny and morphology of the entire vertebrate series, it de- 

 serves detailed study. 



The proboscis and proboscis sheath, from all present embry- 

 ological evidence, seem to arise as correlated structures. The 

 former according to Hubrecht and Salensky is formed as a 

 tubular ectodermal invagination of preoesophageal origin, 

 which becomes surrounded by the sheath that has become 

 constricted off dorsally from the mesoderm or more probably 

 from the endoderm, though exact CAddence on this important 

 point is still lacking. Now, in many of the Turbellaria that 

 show decided affinity to the Rotifera though on more advanced 

 and derived lines of their own (p. 421), there are formed above 

 or behind the mouth a coecal oesophageal invagination and a 

 central proboscidiform swelling, both of which may attain 

 relatively great size and lie at right angles to or above the 

 mouth, while tlie inner walls of the fonner become strengthened 

 by muscular fi})ers that may aid in evertion and retraction of 

 the proboscidiform part. Now were such a coecal sheath 

 extended backward parallel to the alimentary canal, and were 

 its orifice to open along with the oesophagus by a common 



