VERMES. 585 



prae-oral lobe, as well as of the contractile cords to the oesophagus. The 

 fact that a species of Balanoglossus has a larva differing very much from 

 Tornaria and belonging to a simpler type, but that both larvae agree in the 

 mode of origin of the coelome as five archenteric pouches, makes it probable 

 that the Echinoderm characters of the Tornaria are not primary. The 

 mode of origin of the coelomic spaces is however a point in which the 

 Enteropneustan larvae contrast also with the Trochosphere. The per- 

 manent coelome of the segmented Vermes is a schizocoele, which developes 

 as a series of splits in the mesoblast of each of the somites, into which the 

 two mesoblastic bands divide on either side of the body. There is some 

 doubt however whether the schizocoele in these instances is or is not 

 to be looked on as an abbreviated form of development of an enterocoele as 

 it is in Vertebrata. The nature of the two head cavities is a point still 

 unsettled. They may be (i) archicoelic, and appear to be so in Archi- 

 Annelida, or (2) schizocoelic, as described by Kleinenberg in Lnmbricus 

 trapezoides \ 



Asexual reproduction occurs in some Chaetopoda either by the for- 

 mation of new intercalated somites, the growth of one of them into a 

 head, followed, sooner or later, by fission ; or by regeneration of the organism 

 from a few detached somites. Some Nemertea may be similarly regenerated 

 from fragments of the body. Simple fission occurs in a few Turbellaria 

 with the formation of either chains of individuals, or of two individuals 

 which separate at once. The formation of proglottides in Cestoda is 

 probably not to be regarded as an instance of gemmation. The asexual 

 origin of certain generations of digenetic Trematoda from a single cell 

 belongs to a different class of phenomena ; see General Introduction. 

 Alternation of Generations is observable in a few Chaetopoda, in all digenetic 

 Trematoda, and some Nematoda. Most Hirudinea, all Trematoda, and 

 Cestoda, a very large portion of Nematoda, and all Acanthocephala are 

 parasitic, and with the exception of Hirudinea and Monogenetic Trematoda, 

 endo-parasitic. Isolated instances of either commensalism or parasitism 

 occur in Chaetopoda, Nemertea, Tttrbellaria, and Rotifera. 



Thirteen classes are distinguishable among Vermes. Of these (i) 

 the Enteropneusta are completely isolated, but in the organisation of the 

 adult certain resemblances may be traced to the Chordata. The Chaeto- 

 poda (2) constitute a well-defined class 2 , to which (3) the Archi-Annelida 

 are closely allied. The Gephyrea (4) are most probably to be regarded as 



1 The difficulty as to the coelomic spaces of the head hinges on the following points: (i) that 

 there is primitively a forward growth into the head of mesoblast from the trunk ; (2) that this growth 

 becomes more and more pronounced in the higher forms, and (3) takes place at, relatively speaking, 

 an earlier period of larval or embryonic existence. Hence the coelomic spaces in question may have, 

 and rightly have, different values assigned to them in different instances. 



2 The, Oligochaeta among Chaetopoda ought perhaps to be separated entirely from the Poly- 

 chaeta on account of the peculiarities of their generative ducts. 



