25 



Sedgwick* have advocated the derivation of all the higher 

 Metazoa from a form having the aperture pulled out at right 

 angles to the axis, and more or less dilated at its angles, as in 

 many of the Actinozoa, and characterised also by having its 

 enteron prolonged into a series of radially placed diverticula 

 resembling the intermesenteric spaces of the Actinozoa. 

 Such a structure might readily be produced by modifications 

 of Gastrea (see fig. 13), and there are strong arguments in 



Fig. 13. Hypothetical forms shewing stages in the conversion of the Gastrula 

 into a primitive worm. A, Gastrula. B, Elongation has taken place along a line at 

 right angles to the long axis of the Gastrula, and the opening is becoming reduced 

 to a slit dilated at the ends. C, Further elongation has taken place, and the original 

 Gastrula opening is now converted into two apertures (the mouth and the anus), 

 united by a curved tube, formed by the archenteron of the Gastrula. D, The anus 

 has moved further back, and the alimentary canal has become longer and straighter. 

 E, The diffused nervous system has commenced to be concentrated in the anterior 

 part of the body in front of the mouth, and as two longitudinal bands running 

 along the sides of the body, a, anus; in, mouth; p, prse-oral lobe; g, supra- 

 cesophageal ganglion ; d, lateral nerve cord. 



favour of the first ancestral Vermes having had such an 

 arrangement, but it is very improbable that it was related 

 in any w^ay to the similar structures in the Actinozoa, and 

 must have been acquired independently. 



The groups of lower Vermes usually united under the term 

 Platyelmia,f are represented as springing from the main 



* Quart. Journ, Micros. Sc, vol. xxiv, No. xciii., p. 43 (1884). 



+ Lang {Mittheil. Zool. Stat. Neapel, Bd. iii. p. 187, 1882 ; and Fauna 

 und Flora des Golfes von Neapel, XI. Monographie : Die Polycladen, p. 645, 

 1884) tries to show that the lower Vermes have been evolved from the 

 Ctenophora, a course which seems very improbable and will require a great 

 deal of evidence in its favour before it can be accepted. 



