240 CRUSTACEA. 



resemble the adult in form, but are still distinguished by the possession of 

 the frontal band, were described by Bijkmeister as distinct forms under the 

 name of Chalimus. Later, however, F. Muller proved that these forms belong 

 to the ontogeny of Caligus, as had already been conjectured by Kroyer. 



The Lemaeidae are very interesting with respect not only to the parasitism of 

 the larval forms, but to the deformation that occurs in the body of the female 

 after copulation. The metamorphosis ' of Lernaea branchialis has been made 

 known by Metzger and Claus (No. 70). It is probable that the larva which 

 hatches from the egg is a Nauplius resembling that of Achtheres, in which the 

 segmentation of the body of the first Cyclops stage appears to be commencing 

 beneath the cuticle. In these stages there is a free-swimming period, during 

 which a search is made for the first host {Platessa flesus). The youngest forms 

 found attached to the gills still show in all respects the segmentation of the first 

 Cyclops stage (Fig. 113 A). They correspond to the first stage of development of 

 Achtheres. The cephalo-thoracic region is followed by three free thoracic segments 

 and a posterior unsegmented region carrying the furcal processes. Two pairs of 

 well -developed swimming limbs (/-*", f 11 ) can be recognised, one attached to the 

 cephalo-thorax and the other to the first free thoracic segment, as well as a third 

 pair of truncated limbs (on the second free segment). The mouth-parts are 

 already of the true Siphonostomatous type. The upper and lower lips (labrum 

 and paragnatha ?) have fused to form a sucking tube which contains the stylet- 

 shaped mandibles, while the pointed palp-like maxillae are attached to its sides. 

 The first antennae {a') are beset with setae, the second antennae (a"), as well as 

 the anterior maxillipedes, are changed into hooks for attachment. The posterior 

 maxillipedes have completely degenerated, a point in which this larva differs 

 from that of Achtheres. 



The later Cyclops stages which succeed each other (Fig. 113 J?) show decided 

 adaptation to parasitic life. A hardened mass of secretion (k), projecting from 

 the head and comparable to the frontal band of the larva of Caligus, brings 

 about the attachment of the larva to the gills of the host ; this permanent 

 attachment precedes the degeneration of the locomotory organs. Almost 

 all the limbs, and especially the swimming limbs (fl, f n ), are now unjointed 

 and truncated ; they have no setae and are immovable. These stages in which 

 independent movement is lost have also received the name of pupal stages. In 

 this pupal condition the segments of the body and the pairs of limbs which 

 were still wanting are developed. A stage can be distinguished with three pairs 

 of swimming limbs and four free thoracic segments ; at this period, in the male, 

 the posterior maxillipede, until now suppressed, becomes distinct; then a further 

 stage is developed with four pairs of swimming limbs ; this last stage leads 

 through another moult to the free-swimming stage in which copulation takes 

 place (Fig. 114 A and B). Apart from the slight segmentation of the abdomen, 

 the body shows the full development attained by free-living Copepoda. The 

 first antennae (a') are now jointed and beset with setae and sensory filaments, 

 the four pairs of swimming limbs (f 1 ,/ 11 ^), with their clothing of setae, are 

 adapted for active swimming, while in the structure of the second antennae {a") 

 and of the mouth-parts, the Siphonostomatous type is marked. The female 

 (Fig. 114 B) is distinguished by a great lengthening of the genital segments, 

 which gives the whole abdomen the appearance of a long, vermiform appendage. 

 The female genital organs are not yet sufficiently developed for the production 

 of eggs capable of fertilisation ; the receptaculum seminis, on the other hand, 

 with the two pores (g) for the reception of the seminal masses from the 



