166 INVEETEBKATA chap. 



cells, and that in the ancestral form the genital cells formed compact 

 packets lying at the sides of the gut, and at maturity burrowed their 

 way out as they do in Nemertinea. In later stages of the race it 

 is supposed that after the main mass of cells were dehisced, a peri- 

 pheral layer was retained and formed the wall of a hollow sac, thus 

 constituting the primitive ooelomic sac, which on this supposition 

 was a " gonocoele." It is further supposed that in course of time 

 what were originally genital cells became modified into longitudinal 

 muscles and excretory yellow cells, both of which in Annelida are 

 formed from the coelomic lining. 



Now this latter theory seems to fit in well with certain facts. It 

 serves to connect the Annelida and the ISTemertinea, and it is 

 undoubtedly true that at the period of sexual maturity in Polygordius 

 and in many Polychaeta the coelomic cavities become absolutely blocked 

 up by the mass of genital cells which have been proliferated from their 

 walls. But it is a little disconcerting to find that, whereas, according to 

 theory, the coelomic cavity should not appear till after the main mass of 

 the genital cells has been dehisced, and that the surviving cells should 

 then be converted into peritoneum and peritoneal muscles, in actual 

 development, as a matter of fact the cavity appears first and the 

 peritoneal muscles are differentiated a long time before there is any 

 trace of genital cells. Further, when these genital cells finally do make 

 their appearance and are dehisced, far from the worm taking on a new 

 lease of life, which, according to the theory, must have constantly 

 occurred in some ancestral stock, the animal dies and all its tissues 

 disintegrate. Why the ancestral animal should wait to form muscles 

 till the main purpose of its life has been fulfilled, and what it used for 

 muscles in the meantime, are questions very difficult to answer on 

 this theory. 



The theory, however, as we have already said breaks down when 

 other groups of the animal kingdom are studied, unless we are 

 prepared to assume that a fundamental organ with essentially 

 constant character like the coelom, originated from totally different 

 rudiments in different groups, a view which would be subversive of 

 all the recognized principles of reasoning in comparative anatomy. 

 When finally we consider, as all zoologists allow, that the Annelida 

 are derived from Coelenterata, and when we observe that in Scyphozoa, 

 Actinozoa, and Ctenophora, the primitive gut tends to be divided 

 into a central digestive portion and peripheral branches whose walls 

 give rise to muscle cells, and in some of which the genital cells 

 ultimately appear, it seems to us that there can be but one opinion 

 as to which theory of the coelom is the more inherently probable ; 

 indeed it is only in consequence of the myopic concentration of 

 attention on the facts of development in a limited number of groups,- 

 and the neglect of the facts of development in other groups, that 

 the gonocoele theory ever has obtained any vogue. 



The most characteristic feature of Annelida, next to the segmented 

 coelom, is the nervous system, consisting of brain, collar, and ganglion- 



