Hatschek, whose observations on Polygordius and Echiurus are followed in the text, doubts this fact. At any rate such an origin is not primitive. The division of the digestive tract into stomodaeum, archenteron and proctodaeum, e. g. in Eupomatus, is probably typical. In the larval Serpula, however, as described by Conn (Z. A. vii. 1884), the blastopore lengthens out antero-posteriorly, closes, and at its two opposite ends appear the mouth and arms; the middle region corresponding to the median ventral line joining the two. It is possible, however, that an epiblastic stomo- and procto-daeum may be formed subsequently. Compare Hatschek's account of the formation of the stomodaeum in Eupomatus (loc. cit. supra). The Trochosphere of the last-named possesses a pair of otocysts, and, like all Serpulid larvae, an adanal vesicle formed by a vacuolated ectoderm cell according to Hatschek, by an endoderm cell according to Connprae-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 permanent 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 Lumbricus trapezoides 1.
1 The Chaetopod larva (of a Clymenid ?), known as Mitraria, has a remarkable resemblance to a Pilidium. See Metschnikoff, Z. W. Z. xxi. 1871, p. 233.
Asexual reproduction occurs in some Chaetopoda either by the formation 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, Turbellaria, and Rotifera.
Thirteen classes are distinguishable among Vermes. Of these (1) the Enteropneusta are completely isolated, but in the organisation of the adult certain resemblances may be traced to the Chordata. The Chaetopoda (2) constitute a well-defined class 2, to which (3) the Are hi-Annelida are closely allied. The Gephyrea (4) are most probably to be regarded as highly modified Chaetopoda, but it is by no means certain that the three families of Gephyrea should be retained within the limits of a single class. The affinities of (5) the Hirudinea are doubtful: they possess in development mesoblastic somites similar to those of Chaetopoda, as well as provisional renal organs. Their vascular and reproductive systems are peculiar. The nephridial rete found in certain genera is perhaps a primitive feature. Classes 2-5 are sometimes grouped together as Annelida. The Rotifera (6) are an isolated class; they represent in all probability a highly specialised form of Trochosphere, and retain a primitive type of nephridia. The Netnertea, Trematoda, Cestoda, and Turbellaria are sometimes grouped together as Platyhelminthes. The Nemertea (7) are a very distinct class.
The possession of an anus, of a system of vessel-like coelomic spaces, the characters of their nervous, nephridial, and reproductive systems, distinguish them from the other three classes named. The Trematoda (8) and Ces-toda (9) are modified by parasitism, but in many respects, e. g. in the character of the nervous, nephridial, and reproductive organs, resemble (10) the Turbellaria. It is uncertain how far the last named class is to be regarded as degenerate, or the reverse. The larva of some Polyclads is beyond doubt primitive. The Chaetognatha (11), Nematoda (12), and Acanthocephala (13), have sometimes been grouped together as Nematel-minthes. They are, however, perfectly distinct, not only one from the other, but as far as can be judged from all other classes of Vermes, and the last mentioned of the three is most profoundly modified by parasitism.
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.
For a discussion on larval forms, see Balfour, Comp. Embryology, ii. p. 297 et seqq.; for the development of a typical Trochosphere larva, Hatschek'On Eupo-matus uncinatus) Arb. Zool. Inst. Wien, vi. (I), 1885; and for points of interest (origin of oesophageal commissure, mesoblast of head, circular nerves of ciliated rings), Id. ibid. 'On head of Polygordius?