112 Dr. F. Muller on the Metamorphoses of the Fraivns. 



apical half, and beset with long bristles; this is almost con- 

 stantly in a whirling motion. • w j 



In the tail, the lateral plates are now moveably articulated 

 upon a short basal joint and beset with long plumose setae; the 

 middle piece (the seventh abdominal segment) appears to be 

 longer and narrower, as though the two divergent branches had 

 been pressed together to almost complete amalgamation; the 

 setse of the Zoea are retained in their full number, but contracted 

 into short spines. The anus is situated at the base of this last 



segment. 



About the same time a considerable alteration of the heart 

 takes place ; it acquires four new fissures for the entrance of the 

 blood, and internal muscular trabeculaj. 



In this Mysis-like form our larva was observed from scarcely 

 2 mill, to 4-5 mill, in length. During this period the auditory 

 organs, the pincers, and ambulatory feet are developed, and the 

 rudiments of the mandibular palpi, abdominal feet, and branchiae 

 make their appearance. 



The flagella of the antennae become elongated and divided 

 into joints; in animals of 4 to 4-5 mill, in length the two flagella 

 of the inner antennae are three-jointed ; the outer one, which is 

 somewhat shorter, bears about seven bacilli; the flagellum of the 

 outer antenna attains nearly the length of the scale. 



In the basal joint of the inner antenna the auditory apparatus 

 is formed. The lower third of this joint becomes inflated ex- 

 ternally, the swelled portion having a crescentiform anterior 

 margin. In the interior of this inflation an elongated cavity is 

 soon observed (in animals of 3 mill, in length). A httle later 

 there appears in the cavity a globular, strongly refractive otolith, 

 and in the crescent-shaped anterior emargination three or four 

 short, plumose setae, bulbous at the base (fig. 9). The otolith 

 does not appear to lie freely in the cavity, but (as is the case in 

 the tail of Mysis) to be supported by delicate filaments, which 

 issue from a ganglion situated inwards from the cavity. 



The extended spine of the upper lip begins to disappear, but 

 is still recognizable as a minute point in animals of 4*5 mill, in 

 length. The palpi make their appearance on the mandibles, 

 about the time of the formation of the otoliths, in the form of 

 small mamillae, which are soon elongated, but remain unjointed 

 and destitute of setae. 



The pincers {chelce) are indicated, even in animals of 2'8 mill, 

 long, by the still unjointed inner branch of the corresponding 

 three pairs of feet acquiring a small process on the inner margin 

 a little below the apex. In animals of 3*5 mill, in length, these 

 feet are already divided into joints as in the mature animal, and 

 this process (the immoveable finger) attains two-thirds of the 



