PARASITA. 243 



has a long cephalo-thorax, followed by three free thoracic segments and an 

 unsegmented abdomen. In the thoracic region, two pairs of well-developed 

 swimming limbs (p l , p 2 ) and a third rudimentary pair (p 3 ) can be recognised. 

 The first antennae (at 1 ) are cylindrical three-jointed limbs beset with setae. 

 The second antennae are still biramose (at 2 ), but are already changed into the 

 adhering organs of the larva, the longer branch ending in a hook bent like a 

 claw, -while the shorter branch is beset with papillae. The upper lip has 

 united with a channelled lower lip (derived from the paragnatha ?) to form a 

 conical sucking proboscis, from the outer sides of which project the short, conical 

 mandibles, representing the transition from the masticatory jaws of the Cyclo- 

 pidae to the piercing stylets of the Parasita ; here also the palp-like first 

 maxillae are found. These are followed by the two pairs of maxillipedes 

 transformed into adhering hooks (pm l , pm" 2 ), the outer one already assuming 

 a more forward position, and the inner one lying more posteriorly. Among 

 the internal organs, the intestine, the Nauplius eye shifted far back, and two 

 bean-shaped bodies (b, glands ?) at the sides of the latter can be recognised. 



It is probable that the larva, after a short life of free activity in the water, 

 becomes attached as early as this stage to the mucous membrane of the jaw 

 of the perch (v. NORDMANN). The peculiar adhering organ, however, appears 

 to become free, and to be used for its special purpose only after a further moult. 

 In this and two other attached stages, which probably follow through further 

 moiilts, the mandibles shift to a position within the sucking proboscis, while 

 a reduction in the setae on the swimming feet probably takes place. These 

 stages, however, did not come under observation, but in a somewhat later stage 

 which was observed (Fig. 115 C), there was already considerable approxi- 

 mation in the structure of the body to the adult Achtheres. The body has 

 now become almost vermiform, the first thoracic segment having separated from 

 the head and having united with the four following segments to form a sac-like 

 region, at the end of which the small, pointed, furcal appendage can be recognised. 

 The antennae and the mouth-parts already essentially resemble the corresponding 

 parts of the adult. The frontal adhering organ, with the exception of a vestige 

 of its basal section (z), has vanished; on the other hand, a new provisional 

 adhering organ in the shape of a stiff filament (/) has arisen on the outer 

 (anterior) maxillipede (inn 1 ) ; this filament starts from the tips of the fused outer 

 maxillipedes. It is an interesting fact that at this stage sexual differentiation 

 begins to be noticeable. Smaller individuals (young males) have remarkably 

 strong outer maxillipedes (jam 1 ), which are only united at the point of insertion 

 of the adhering filament, and carry a strong terminal hook. When this filament 

 is thrown off in the next moult, these structures give rise to the anterior maxilli- 

 pedes of the male, which remain distinct and function as hooks for attachment 

 (Fig. 115 E). The posterior maxillipedes (pm 2 ) are also very large and each 

 carries a small anchoring hook. In the female, on the contrary, the anterior 

 maxillipedes (pm l ) are rather long and retain the fused condition, ending in 

 a sucking disc (Fig. 115 D) ; the posterior maxillipedes can in the same way 

 be distinguished from those of the male by a large, hook-like, terminal joint 

 With regard to the inner anatomical peculiarities of this larval form, we must 

 first mention the degeneration of the Nauplius eye, which, indeed, is not 

 universal among the parasitic Crustacea. The Nauplius eye is retained in the 

 dwarf males of Chondracanthus and of Lernaeopoda, as well as in many 

 females (e.g., Chondracanthus cornutus). In the posterior cephalic region of 

 the larvae under consideration, at the sides of the intestine, there are two pairs 



