The larvae of the second group (Fig. 117) have a similar 

 body shape and are also equipped with a ciliary covering which is divided 

 into three zones, but which for the most part is more powerfully developed. 

 There are also well -developed glands of the head end opening outside for 

 the most part by three groups of ducts. Usually, the eyes are 



in the number of one fused from two (which is clearly indicated by the pre- 

 sence of two light refracting lenses), more seldom there are two eyes and 

 even more seldom- -four (Diclybothrium, Axine). The buccal opening is 

 somewhat closer to the anterior end than among the larvae of the first 

 type. The pharynx is powerfully developed and is located at a greater 

 distance from the buccal opening, the intestines are circular or sac- 

 shaped, A development of the nervous system and. excretory system is 

 approximately the same as in the first group. The sex system is incepted 

 behind the intestinal sac or in the same fashion as among the larvae of the 

 first group. The attaching armature of the disc consists often to twelve 

 edge hooks or, as an exception, of a smaller number (genus Diplasiocotyle), 

 or the edge hooks can even be absent, but then there are other attaching 

 formations (genus Diplozoon). As a rule the edge hooks of this group are 

 already fully developed during the period of embryonic development and 

 differ from the edge hooks of the larvae of the first group in that their 

 little hook is more elongated, whereas the handle is straight or slightly 

 curved, more delicate and flexible. 



Within the limits of the group of the larvae of the first type we 

 observe differences in the attaching apparatus which have quite regular 

 character. On the one hand, they concern the chitinous armature of the disc, 

 and on the other hand, the appearance of attaching formations supplementary 

 to it- -suckers. As was pointed out earlier for the families of Monogenoidea 

 which have similar larvae, the attachment of adult forms takes place either 

 only with the help of chitinous armature or with the help of the disc --sucker, 

 or with the help of suckers on the attaching disc. It is understood, that be- 

 tween these methods of attachment there are a number of transitions when 

 the worms attach" utilizing different formations at the same time. The most 

 primitive undoubtedly appears to be the attachment with the help of the 

 chitinous armature (alone, nobis) and the most complex for a given group- - 

 suckers. The change in the structure of the attaching apparatus of the larvae 

 develops in the same direction as the adaptation to the attachment among 

 adult forms. The larvae of Dactylogyridae , Diplectanidae, and Tetraonchidae 

 possess only chitinous armature, and the adult forms of this family also attach 

 only with its help. The larvae of Calceostomatidae and Capsalidae possess 

 only chitinous armature also, but their adult forms maintain themselves on 

 the host mainly by means of a disc transformed into a sucker. Tetraonchidae 

 and Tetraonchoididae, which have a larva similar to the more simply or- 

 ganized Dactylogyridae, probably attach themselves with the help of a disc 

 and partially by chitinous armature. The larvae of Sphyranuridae attach 

 themselves with the help of chitinous hooks and suckers in the same fashion 

 as their adult forms. F inally, in the adult condition, Polystomatidae attach 



101 



