THE NERVOUS SYSTEM. DEGENERATION. ATTACHMENT. 399 



The nervous system is always small, exceedingly simple and primitive in 

 structure. Sense organs, such as visual and auditory organs, are absent, or very 

 rudimentary. Even in the cirripeds, the lateral and parietal eyes of the larva 

 quickly disappear, or become functionless. Only in a few tunicates does the 

 parietal eye function to any extent in the adult. A neuro-muscular apparatus, 

 capable of elaborate or varying responses to external stimuli, is never present. 



The mode of life is rigidly prescribed by these conditions. The absence of 

 armored grasping appendages, of well developed sense organs, and of a complex 

 neuro-muscular apparatus, excludes the possibility of elaborate reflexes, of percep- 

 tion at a distance, of pursuit and capture. The inevitable result has been the 

 practically universal adoption of either a sessile, a subterranean, or a parasitic 

 mode of life, depending for food on micro-organisms, or on other finely divided 

 matter sifted from water or soil, or on fluids absorbed from other animals. 



Degeneration. — There is a strong tendency in the entire group toward a 

 retrograde or degenerative development, that appears to be due to some prevalent 

 lack of adequate internal conditions or of materials. It makes its appearance 

 during, or shortly after the larval stages, cutting down the first promises of a clear 

 cut, vigorous organogeny to one that is feeble, blurred, or defective in definition; 

 or one in which important parts are absent. It may manifest itself in the absence 

 of structural detail, in diminished local outgrowths, or in the absence of appen- 

 dages (Amphioxus, enteropneusta, tunicates, chastognaths) . It is seen in the 

 degenerative metamorphosis of tunicate larvas; in the reduction in size, or absence 

 of organs, so common in male cirripeds; and in the progressive disappearance 

 in many parasitic cirripeds of both sexes, of mouth, anus, appendages, nervous 

 system, and alimentary canal; in fact, of practically everything except the integu- 

 ment and reproductive organs. 



In some cirripeds (rhizocephala) , this process is carried so far that if it were 

 not for the presence of the characteristic appendages in the larvae, their identity 

 would be exceedingly difficult, perhaps impossible, to determine. We have 

 merely to assume that the suppression of appendages has been carried a step 

 further back in the ontogeny, to account for their total absence in Amphioxus and 

 the enteropneusta. 



In the rhizocephala, the parts of the body left after the extensive degeneration 

 of organs, and the casting off of the abdomen, acquire a new, almost unlimited 

 power of growth, forming extensive, root-like processes that penetrate in every 

 direction the tissues of its host. In the ectoprocta, there is also an extensive 

 degeneration of organs, similar to that in parasitic cirripeds. That is, the nervous 

 system, appendages, and alimentary canal disappear, or fail to develop, and from 

 the apparently formless remnants, strangely enough, buds are formed, destined to 

 give rise to new and more perfect zoids. It may be that there is some relation 

 between the degenerative, or retrograde development of the tunicates, ptero- 

 branchia, and polyzoa, and this retention and renewal of the power of budding. 



Attachment. — We have seen that many phyllopods are temporarily attached 



