The development of these organs, which in the Protonemertine are but grooves in the epidermis, not far removed from the similar cephalic slits of many Turbellaria, reaches its height in Drepanophorus.
Pharynx, and he sums up their relationship to the Annelids by thestatement that to a certain extent the Nemertines represent Turbellaria which in the course of time have copied certain features of an Annelid character.
Whether this view be adopted or not, and whether the Turbellaria be regarded as nearly related or only remotely connected, there can be little doubt that the Nemertines resemble the Turbellaria more nearly than they do any other group of animals.
The chief peculiarities that distinguish Trematodes from their free-living allies, the Turbellaria, are the development of adhering organs for attachment to the tissues of the host; the replacement of the primitively ciliated epidermis by a thick cuticular layer and deeply sunk cells to ensure protection against the solvent action of the host; and (in one large order) a prolonged and peculiar life-history.
A single or paired accessory gonopore is met with in many Trematodes just as in certain Turbellaria (e.g.
2, D) and canaliculi of the excretory system as in Turbellaria, others again muscle-cells.
Of Turbellaria and Ne- a, anus; ap, apical organ, correspond mertina.
Within this muscular tube lies a parenchymatous tissue which may be uniform (Cestodes) or differentiated into a central or digestive, and a peripheral portion (some Turbellaria), or finally the central portion becomes tubular and forms the digestive sac (Trematodes), while the peripheral portion is separated from it by a space lined in some forms by a flattened epithelium (most Planarians).
The Turbellaria undoubtedly form the most primitive division, as is shown by their free-living habits, ciliation and sense-organs.
As the Turbellaria (Planarians) are the most primitive division of the Platyelmia, the problem of the affinities of this phylum resolves itself into that of the relationships of the Turbellaria.
On the one hand the Turbellaria are considered to be an offshoot of the early Coelomate stock, on the other they are held to be descendants of a simpler two-layered stock.
The Trochosphere-hypothesis (2), (3) is based chiefly on the occurrence in certain Polyclad Turbellaria, of a larval form (Miiller's larva) which is comparable to a certain stage (pro-trochula) in the development of the Trochosphere-larva.
- Dorsal view of Coeloplana to illustrate the similarity between Ctenophora and Turbellaria (X 12).
View (held on other grounds) that the Polyclads are the most primitive of the Turbellaria, is soundly based.
In effect (6) it traces the Turbellaria to small two-layered organisms consisting of an outer ciliated epidermis and a central syncytial tissue.
From such beginnings the evolution of the Turbellaria leads first through the Acoelous forms in which the central syncytium is partly differentiated into digestive, muscular and skeletotrophic tissue, then to the more specialized Rhabdocoela, and so through the Alloeocoela to the Triclads and finally to the Polyclads.