THE TURBELLARIA 17 



-(originally observed by 0. Schmidt), sense organs of peculiar char- 

 acter similar to those which exist in many Annelida, also occur in 

 Microstomidae, Plagiostomidae, and a few others (III. 4 ; VII. 5) ; 

 the pit rests upon a group of ganglion cells, which is directly or 

 indirectly connected with the brain; these may, perhaps, be the 

 precursors of the elaborate " cerebral organs " of the Nemertines. 



Another character of the Nemertines, the "proboscis" (as 

 Leuckart first pointed out), is foreshadowed in the family Probos- 

 cidae (VI. 1-7). In all Turbellaria the anterior end of the body is 

 .specially provided with tactile hairs; and in several genera this 

 region is capable of a slight invagination. In this family the 

 simplest stage is represented by Pseudorhynchus, in which the 

 anterior end of the body is deprived of cilia, somewhat prolonged 

 .and capable of freer movement than usual by the action of numer- 

 ous short muscles. In the remainder of the family this " snout " 

 is permanently withdrawn into a pouch, which usually lies at the 

 tip of the body, but may be subterminal and ventral (Hyporliynchus, 

 Fig. VI. 5); the snout or proboscis now consists of a mass of muscles, 

 .and is capable of protrusion, while withdrawal is performed by four 

 long retractor muscles. The " frontal organ " of Acoela was at one 

 time thought to be a similar sense organ; but v. Graff (24) has 

 .shown that it is entirely glandular (Fig. V. 2), containing neither 

 muscle nor sense cells. 



The sub-order Khabdpcoela possess an intestine which retains 

 much of the ancestral character; it is a straight and simple 

 elongated sac (Fig. III. 2, 4). The intestine may be constricted 

 by the ripe gonads, and by the development of dorso-ventral 

 muscles to form incomplete " septa " ; this leads to a lobing such 

 as occurs in Alloiocoela (Fig. VIII.), which reaches its highest phase 

 in Bothrioplana, where the lobes become long and regularly placed ; 

 the large pharynx of this genus, placed posteriorly, indents, as it 

 were, the gut, which now passes on each side of it, and has quite 

 the appearance of that of a Triclad. 



The mouth retains its primitive position near the anterior end 

 in a considerable number of forms, though it may occupy any 

 position on the ventral surface ; this mouth leads directly into the 

 digestive parenchyma (hypoblastic syncytium) in Acoela, though 

 frequently the epidermis is slightly invaginated, and the muscular 

 coats are here thicker (Fig. V. 7, 8). This is ontogenetically what 

 happens in the other groups in which a distinct pharynx is formed. 

 This pharynx has various shapes within the group of Turbellaria, 

 which have received various names (Fig. VI. 8-13); the two chief 

 varieties distinguished are : (a) Ph. bulbosus, in which the muscles 

 of the pharyngeal wall are surrounded by a distinct sheath, separat- 

 ing them off from the parenchyma. This type of pharynx, which 

 is used for sucking and cannot be protruded far, occurs in the 



