432 Causes and Course of Organic Evolution 



In cj^clostomes the suctorial character has become, so to say, 

 typical of the group. We would even consider that this in 

 large part has started the semi-parasitic habits in fresh-water 

 ancestors of some ancient type, that is now more pronounced 

 in the fresh-water or temporarily salt-water Petromyzon, as 

 well as the almost complete parasitism along with the marine 

 life of Myxine. 



Apart from the protrusible gullet, the ahmentary system 

 in the Rotifera, and in some rhabdocoel Turbellaria, shows 

 a straight alimentary canal made up of muscular pharynx, 

 glandular stomach, short intestine, and in most females of 

 the former group a posterior anus. A varying degree of ad- 

 vancing complexity is seen in the nemerteans from a simple 

 tube (Malacobdella) to one with shght lateral enlargements 

 at intervals as in Carinina, and from this to types in which 

 the coeca are expanded and even subdivided. This is com- 

 bined in many cases with similar annular disposition of the 

 transverse blood vessels, nerve threads, and reproductive sacs, 

 as worked out by Mcintosh and Blii'ger wdth great care. This 

 also clearly prepares for or anticipates the like segmentation 

 observed throughout the vertebrates, while some of the ali- 

 mentary coeca may have been the starting points for the pairs 

 of secreting accessory glands of it. 



But the nervous system, next to the proboscidial system, 

 seems exceptionally suggestive; in fact it is even more con- 

 firmatory of the vertebrate relation. For it shows homological 

 details throughout, such as neither Balanoglossiis, the ascid- 

 ians, nor Amphioxus show. 



In the Rotifera a relatively large paired cerebral ganglion 

 is the center of origin for nerves to the supposed olfactory 

 organ, to the tactile organs, to the eyes, and other cephalic 

 sense-centers. A commissural ring connects this with a ven- 

 tral pair of ganglia. But from the cerebral mass according 

 to Zelinka two strong nerves arise that branch into feeble 

 dorsal, into strong lateral, and into strong ventral threads 

 which pass to the foot, the muscles, etc. 



In many fresh- water rhabdocoel turbellarians, there is a 

 pair of fused and well-developed cerebral ganglia from which 

 two strong ventral, two rather strong lateral, and two weaker 

 paired dorsal nerve-threads are given off. But we would 

 regard the last of these as of extreme importance. For by 

 gradual approximation and ultimate fusion in higher rhab- 

 doccels and in nemerteans, as their function in relation to 

 the eye on the one hand and the anterior external muscles 

 on the other became increasingly accentuated, they furnish 



