182 MARINE AND FISHERIES 



5 GEORGE v., A. 1915 



In the smaller forms, e.g., those shown in Fig. Ic, the movements are con- 

 fined to irregular contractions of small amplitude of the whole body in a longitu- 

 dinal direction. 



Anatomy of Larvae of Different Sizes. 



The smallest specimen investigated by means of serial paraffin sections meas- 

 ured 0*29 mm. in diameter by about 0-25 mm. in length. The suckers show 

 narrow spindle fibres, two zones of nuclei and circular muscle-fibres on the inside 

 and outside of the spindles, all characteristic of the adult P. ambloplitis as described 

 by Benedict. The invagination chamber is large and contains mucus. The 

 measurements are considerably less than those given below for an older larva, the 

 end-organ being 0-058 mm. in diameter and the suckers 0-084 mm. The 

 former is essentially similar in structure to that of the older plerocercoids. A few 

 nuclei are found lying within the basement membrane, and there is found a cross 

 of large muscle-fibres in the parenchyma behind the organ. This parenchyma is 

 very loose and open, especially immediately behind the end-organ (here situated 

 more posteriorly than the suckers, since the scolex is invaginated). Longitudinal 

 muscle-fibres are few, but there are many nucleated anlagen in that area. The 

 cuticula is thin and the cuticular muscles are poorly differentiated. The caudal 

 vesicle gives off two main branches. Parenchyma cells surround these branches, 

 as described below and extend for some distance out on secondary branches, 

 thus suggesting the origin of the excretory vascular system (cf. Braun, '94-'00). 

 Only a few parenchymatous spaces are to be observed, and the connections between 

 them and the branchlets of the caudal vesicle are not evident. 



Later the parenchymatous cells grow and take on a more definite stellate 

 appearance showing their fine processes distinctly while the muscle-fibres become 

 more strongly developed from the anlagen in the parenchyma. 



Larva, 0.7 mm. in length, PI. XIX, Fig. Ic. 



At this stage of the development the cuticula measures Sjj. in thickness, just 

 In less than that given by Benedict for the adult P. ambloplitis. The tube leading 

 from the invagination chamber to the exterior has a diameter of 48/i including the 

 cuticula itself, which is here deeply incised. Around this tube the circular muscles 

 are well developed whUe the longitudinal fibres are very numerous and quite 

 large. The suckers are llO/x in diameter and show at their centres spindle fibres 

 32^ long, the rest of the musculature being well-developed in plerocercoids of this 

 size. The crenulated cuticle lining the cavities of the suckers is Z/i in thickness. 

 The end-organ, which, so far as structure is concerned, seems to be as well-devel- 

 oped as that of the plerocercoid described below, has a diameter of 0-150 mm. 

 and a length of 0-135 mm,, which measurements show that it is proportionately 

 much larger than in an older plerocercoid (vide infra). The caudal vesicle, QOft 

 in length, is forked for a distance of 15/i, and this forked portion is lined with a 

 continuation of the cuticula applied to the inside of the vesicle itself. Small ab- 

 sorptive cells are grouped around the vesicle in the typical manner, but the cuti- 



