200 COMPARATIVE ANATOMY CHAP. 



proboscis retractor consists of longitudinal muscles, which, running 

 inside the proboscis sheath, are attached on one side to the base 

 (posterior , wall) of the proboscis sheath, and on the other to the apex 

 of the proboscis. By its contraction the proboscis is invaginated. 

 The proboscis retractor is continued at the posterior end of the pro- 

 boscis sheath into the two muscular retractors of the proboscis 

 sheath, which run through the body cavity, one being dorsal and the 

 other ventral, to attach themselves to the dermo- muscular tube. 

 These retractors hold the proboscis sheath in its place. At the base 

 of the proboscis sheath there arise further two occasionally muscular 

 strands, the retinaeula, which run laterally through the body cavity 

 into the body wall, and carry back within them lateral nerve strands 

 which come from the cerebral ganglion lying in the base of the pro- 

 boscis sheath. 



Y. The Intestinal Canal. 



The intestinal canal in the Worms is, in general, well developed 

 and provided with an anus. Only in the endoparasitic Acantliocephala 

 every trace of an intestinal canal has disappeared. In the males 

 of the Rotatoria also, and the males of certain species of Dinophilus, 

 the intestine is more or less completely degenerated. The intestine 

 of the dwarf male of Bonellia, which lives parasitically in the female, 

 is without mouth or anus. In the sexually mature Gordiidce also the 

 mouth is closed by an overgrowth of the cuticle. An anus is wanting 

 in various Nematoda, such as Mermithidce, Ichthyonema, and Filaria 

 medinensis. In the hermaphrodite generation of Allantonema mirabile 

 the intestine is quite reduced. A more or less far-reaching reduction 

 of the intestine can also be found in other Nematoda, e.g. Atractonema, 

 Sphcerularia. The intestine ends blindly in the Testicardines among 

 the Brachiopoda, and in Asplanchna among the Rotatoria. All these 

 defects and degenerations represent a derived condition, in contrast 

 with the well-developed intestine which is provided with an anus. 

 The degeneration can for the most part be ontogenetically established. 



The walls of the intestine consist almost everywhere of 2 layers, 

 an outer muscular layer, which we might name the intestino-muscular 

 tube in contradistinction to the dermo-muscular tube, and an inner 

 epithelial layer turned towards the intestinal lumen. 



We can, from an ontogenetic point of view, distinguish three 

 divisions in this intestine, the first two being already known to us in 

 the Ccelenterata and Platodes. (1) The fore-gut which proceeds from 

 the stomodaeum of the larva or embryo. Its epithelium is of ecto- 

 dermal origin, and it seems chiefly to supply the various adaptations 

 for seizing food, for reducing it into smaller pieces, and for passing it 

 on further (pharynx, jaws, teeth). (2) The mid-gut comes from the 

 mid -gut (mesenteron) of the larva; its epithelium is of endodermal 

 origin. It forms the principal digesting portion of the intestine. (3) 



