REGENERATION. PARASITES. 101 



features which are only explicable on the view that injuries 

 are being repaired. Thus individuals are found with a very 

 small proboscis and collar, or with an imperfectly formed collar 

 indicating that these parts are being regenerated. 



It is asserted that in the regeneration of the collar of Ptychodera flava 

 the collar nerve cord is formed as an open groove which is gradually con- 

 stricted off, and that in the regeneration of the proboscis of the same 

 species the proboscis pores may be equal in sir.e. The first fact has its- 

 counterpart in some tornaria larvae in which the medullary cord arises by 

 the constriction off of an open groove, but the latter has no counterpart 

 so far as is known in the early developmental history, no tornaria being 

 known with two water-pores. 



Various parasites (Gregarines, flagellate Protozoa, Trenia- 

 todes, Nematodes, etc.) are found in the Balanoglossida, the 

 most remarkable perhaps being a parasitic vqpepod. Jmbalaripi' 

 glossi sometimes found in the genital pleurae. 



The Balanoglossida are remarkable for the ^oniplexity of their 

 structure in contrast with the simplicity of their mode of life and 

 the absence of organs of special sense. There are no known organ- 

 isms, whether animal or vegetable, at present living, which pre- 

 sent such complexity of organization combined with such simplic- 

 ity of habits. They possess all the three features of the Chordata 

 (see vol. 2, ch. i.) : namely a notochord developed from the dorsal 

 wall of the enteron ; a tubular central nervous system ; and 

 paired branchial apertures leading outwards from the anterior 

 part of the alimentary canal. In addition they possess complex 

 vascular and muscular systems, and a coelom which, largely 

 filled up by muscular and connective tissue in the adult, consists 

 of three main divisions, viz. an unpaired chamber in the pro- 

 boscis, two paired chambers in the collar, and a similar number 

 of chambers in the trunk. 



All the mesodermal structures including the gonads develop 

 from the walls of the coelomic sacs, but no relation has as yet 

 been traced between the cavities of the gonads and those of the 

 coelom. There is no relation between the repetition of any of 

 the organs and that of the coelomic sacs. Indeed all the repeated 

 structures of the adult, viz. the gill- slits and the gonads occur 

 in the region of the posterior or trunk sacs. Although there is 

 no regularity in this repetition it is interesting that it should 

 occur here, for it is these posterior sacs which in Amphioxus 



