THE TREMATODA 83 



with the cuticle ; and at the same time, the basement membrane 

 has been pushed upwards against the cuticle ; the cells have, so to 

 speak, slipped down through this membrane. 



It is worthy of note that Max Braun has observed a definite 

 external layer of cylindrical cells in the lateral suckers of Nitzschia 

 and Epibdella ; they are not covered by a cuticle, which stops 

 abruptly against the cells. 



The constitution of the parenchyma (or mesenchyma) also 

 presents difficulties of elucidation; by most authorities it is 

 regarded as consisting of more or less highly vacuolated, granular, 

 nucleated cells, the extent of the vacuolation differing in different 

 genera, and in different parts of one and the same worm (see 

 Walters, 47). On the other hand, it has been more than once 

 suggested, and recently again by Blochmann, that these vacuoles 

 are intercellular, the cells themselves being extremely branched. 



The musculature retains the same arrangement as in Tur- 

 bellaria, but the large " myoblasts " give rise not to one muscle 

 fibre, but to many. 



In the Trematoda the necessity for fixation which is effected 

 primarily by the posterior sucker, appears to have led to a forward 

 movement of the generative pores in most forms, as well as of the 

 excretory pore in Heterocotylea, and in the former point the Class 

 contrasts with the Turbellaria. The peculiar secondary adhesive 

 apparatus of Holostomidae is clearly antagonistic to this forward 

 position, and we find the genital pore at the posterior end, which 

 must be regarded as secondarily acquired by them. 



The mouth, however, retains its ancestral position at the 

 anterior end of the body. It leads into a buccal cavity, upon 

 which follows a pharynx bulbosus. The intestine has lost its 

 primitive, sac -like shape, owing to the forward and central 

 position of the generative organs. Coincidently with the great 

 development of these, the dorsal and ventral walls of the intestine 

 have coalesced as in many Turbellaria (cf. the origin of radial 

 and circular canals in Medusae), and in great part disappeared, so 

 that two main limbs, one on each side, remain. Nevertheless, 

 this obliteration of the central region has not occurred in the 

 Aspidocotylea or Gasterostomum, and only incompletely in many 

 Heterocotylea, where the two forks are united by transverse caeca 

 (as Polystcmum). In this and other instances, too, the intestine 

 resumes its median position behind the gonads (Fig. II. 2, 4). The 

 reticular gut of the Polystomea is therefore more primitive than the 

 simple, bifurcated intestine of such forms as Calicotyle, Tristomum, 

 and most of the Malacocotylea. This view is supported to some 

 extent by the fact that in the young Polystomum the central intestine 

 is a simple sac which only exhibits its characteristic form during 

 the appearance of the gonads (Fig. V. 7). In other forms, also, 



