of the Fishery Board for Scotland, 



143 



middle : a joint was not satisfactorily made out. A swelling has 

 appeared on its distal half. This is the rudiment of the single-jointed 

 process of the Megalops. The ganglion is in the distal half of the joint. 



Megalops (Fig. 29). The antennule is now a very different organ. 

 It is 2-branched. The first joint of the appendage in the I. Zoea 

 is no longer to be made oat ; it has probably fused with the cephalon. 

 The antennule consists of seven joints, a large basal joint, triangular in 

 shape, representing the broad proximal half of the appendage in lY. 

 Zoea, and six other joints which represent the distal half of the long 

 joint in IV. The second and third joints are broad, and contain within 

 them the ganglion, now divided into two. The third joint has a broad 

 distal end, from the projecting corner of which a single- jointed 

 appendage arises. The latter joint is tapered, constricted at its middle, 

 and bears four terminal smooth hairs; and two other shorter hairs arise 

 from the joint as shown in the figure. It is the endopodita. 



Distal to the third broad joint there are four narrower joints : the 

 fourth, fifth, and sixth are short, the seventh a long joint constricted 

 at its middle. There are three main groups of aesthetascs attached to 

 the last three joints. On the fifth joint there are three ; on the sixth, 

 five (in one case four) ; and on the seventh a group of three about the 

 middle of the joint, and a single one a little short of the end of the 

 joint. On the end of the joint there is a long sparsely ciliated hair. 



There are several short hairs distributed over the other joints : these 

 are shown in the sketch. 



The first joint contains the auditory organ, for a minute description 

 of which see Hensen's* and Prentiss't papers. 



The sensory tubes are connected by tubes with the ganglion, which is 

 not single but composed of different ganglia, which appear to serve 

 different aesthetascs or groups of aesthetascs. The compound ganglion is 

 situated in the second and third joints. From the position of the 

 ganglion we are able to homologate the different portions of the 

 antennule of the Megalops with that of the IV. Zoea. The lower half 

 of the long pear-shaped joint becomes the first joint of the Megalops ; 

 and the remaining six joints and the short appendage from the third 

 joint are derived from the distal half of IV. The little 1 -jointed 

 appendage is the endopodite : the aesthetasc-bearing appendage is the 

 exopodite. 



In Carcinus we have an exactly similar condition to what is found in 

 Crangon vulgaris.X In the latter the auditory organ is in the first joint ; 

 the compound ganglion is located in the second and third joints, and 

 from the the third joint arise an exopodite [3-jointed] bearing groups 

 of aesthetascs, and an endopodibe [2-jointed] bearing hairs only. In 

 Crangon, however, the antennule in the VI. stage (Megalops) is straight, 

 whereas in Carcinus the antennule is bent over on itself. 



1. (Figs. 18, 20, 21.) In the first young stage the condition is practi- 

 cally identical with that of the Megalops, e ^cept that with increase in 

 size the joints are more numerous. The first three joints are unchanged, 

 except in size, but the terminal joint of the exopodite is now segmented, 

 and the 1 -jointed endopodite is now 2-jointed. That is, the antennule has 

 now eight joints, with a 2-jointed endopodite rising from the third 

 joint. The endopodite and exopodite are bent down towards the third 

 joint. 



* Hensen, "Studien liber das Grehororgan der Decapoden," ZeitscJir. f. ^vissenschaft, 

 Zoologie, Bd. 13, p. 319 ; 1863 ; 4 Taf. 



t Prentiss, "The Otocyst of Decapod Crustacea: its Structure, Development, and 

 Functions," Bull. Mus. Gomva^r Zooloqy, Harvard vol. xxxvi.. No, 7, 10 pi. Cambride-e 

 U.S., A., 1901. ■ ■ ' 



X Op. cit. 



