306 J. W. SCOTT MACFIE. 



about three-quarters of it is chitinised, the hair-tuft beign triple. The pecten is 

 composed of 12 to 18 scales. The anal segment is more extensively chitinised, and 

 the relative length of the papillae remains about the same. The dorsal hairs on the 

 anal segment have now begun to subdivide, the dorsal pair being usually bifid in this 

 phase. 



Fourth Phase. 



In the final phase the length of the antenna is 18 to 20 units ; the plume, a single 

 simple hair, is situated just above the middle point, and there are no spines on the 

 shaft. The labial plate has 13 or 14 teeth on each side. The thorax is much ^nder 

 than the head, and the ventral hooks are large and powerful. The number of scales 

 in the comb varies from 8 to 12. Practically the whole length of the siphon is 

 chitinised, the length being ratlier less than twice the basal diameter. There are 

 14 to 20 scales in the pecten. The hair-tuft on the siphon has 3 to 5 branches. The 

 anal segment is highly chitinised, and is often broader than long. The papillae are 

 at this stage more than twice as long as the anal segment. The four hairs on the dorsal 

 edge of the anal segment are more freely subdivided, the dorsal pair having from 3 to 5 

 branches, and the ventral being usually bifurcated. The sub-siphonal plume is made 

 up of several sub-plumose branches. 



Since such marked morphological changes occur during the larval stage it is hardly 

 necessary to point out that for purposes of identification the phase of development to 

 which the larvae have attained must be known. The first phase is a brief one, and 

 could only be secured by hatching eggs, and the second and third phases are only 

 transitional and are not always easy to identify by a superficial examination. It 

 would therefore be best for purposes of classification to deal only with larvae in the 

 last phase, the fourth, and these fortunately are always easy to recognise. 



Certain characters are constant throughout the larval period, such as the short and 

 simple antenna with a rudimentary plume, the presence of a comb consisting of a 

 single row of scales, the rather short and stumpy siphon with a well developed pecten 

 and a small hair-tuft a little beyond its middle point, and the presence of long anal 

 papillae with bluntish ends. These one may suppose to be very old and well 

 established characters. 



The morphology of other features seems to undergo a regular and progressive 

 development as the larva matures. For example, the antennal plume moves slightly 

 but regularly upwards in successive phases ; the spines on the shaft of the antenna 

 are gradually reduced ; the longest of the hairs at the end of the antenna grows 

 relatively shorter ; the teeth on the labial plate increase in number ; the thorax 

 broadens until it is much wider than the hea.d ; the ventral thoracic hooks grow 

 larger and more powerful ; the siphonal, sub-siphonal, and anal plumes subdivide ; 

 the number of scales in the pecten increases ; the anal segment becomes broader ; 

 the anal papillae double in length at each moult, and the hairs on the dorsal edge of 

 the anal segment subdivide. It is features such as these that in their final form one 

 might expect to be characteristic of the species, and from amongst them one might be 

 able to select some that would be useful for identification. 



