186 COMPARATIVE ANATOMY CHAP. 



Among the worms, generally near the Rotatoria, we place the little group of the 

 Gastrotricha. Small animals with ciliated ventral surface ; with pointed processes 

 arranged in longitudinal rows on the dorsal surface. Posterior end running out into 

 two lateral points. Intestine straight, the mouth followed by a muscular pharynx. 

 Anus at the posterior end. Hermaphrodites. Nephridia insufficiently known. 

 Blood-vascular system wanting. Ichthydium : principally inhabitants of fresh 

 water. The Echinodera, which are often placed near the Nematoda, are minute 

 marine animals without cilia, with ringed bodies ; with setse. Inner organisation 

 not sufficiently known. 



The phylogenetic relations of the worms are still a subject of much dispute. 

 There are many different views. The Nemertina form a natural well-demarcated 

 class which in many points of their organisation (nervous system, excretory 

 system, absence of body cavity) recalls the Turbellaria, but they are raised 

 above the Platodes by the possession of a blood-vascular system and an anus. The 

 pseudo -metamerism of the Nemertian body is similar to that of certain Turbellaria 

 ( Triclada). The systematic position of the Nemathelmia is quite uncertain. Their 

 ancestors were probably more highly developed worms, in whom adaptation to 

 the parasitic mode of life has led to degeneration. Perhaps the Gordiidce among 

 the now living Nematoda are the nearest to the ancestral form. The Annulata 

 form a large group extraordinarily rich in forms, in which the typical segmented 

 condition of the body may be regarded as primitive. The Myzostomidce, Echiuridce, 

 many simply organised Chcetopoda, and in some respects the Hirudinea, are to be 

 regarded as one-sidedly developed, partly simplified or degenerated forms. Opinions 

 are very divided as to the racial history of the whole class. Many investi- 

 gators, among whom we include ourselves, hold the segmentation (metamerism) of 

 the Annulate body to be a continuation of the pseudo-metamerism of animals 

 resembling the Turbellaria and the Nemertina. Others consider the Annulate body 

 as a sort of animal stock, which has arisen by axial budding. They see in the 

 Rotatoria the nearest approach to the unsegmented (not budding and not stock 

 forming) racial form, while we, on the other hand, are inclined to consider the wheel 

 animalcules as simplified animals which attain sexual maturity at an earlier stage 

 of development, so that they now no longer rise above the degree of organisation of 

 a young Annelid larva. The class of the Prosopygia falls into a few natural orders 

 to some extent sharply distinguished from each other ; their organisation is in many 

 respects quite comprehensible, if we refer it back to an old adaptation to a more or 

 less attached mode of life, and bear in mind the degenerating action of shell, case, 

 or tube formations on the bodies of segmented worms originally more highly de- 

 veloped. The systematic position of the Ohcetognatha also is very uncertain. They 

 are, perhaps, best considered as Annulata with a small number (3) of segments. 

 It has till now been impossible finally to decide the systematic position of Gastro- 

 tricha and Echinodera. 



I. Form of Body and Outer Organisation. 



The body of the Nemertina is elongated, ribbon-shaped, being more 

 or less flattened dorso-ventrally ; it is ciliated all over the surface, 

 soft skinned, unsegmented, and without outer appendages. The 

 mouth lies ventrally at or near the anterior end of the body, and in 

 the form of a longitudinal slit. In front of it, and generally quite at 

 the foremost end of the body, is the proboscidal aperture. Mouth 



