194 



BULLETIN OF THE UNITED STATES FISH COMMISSION. 



The following measurements show the greatest diameter of the eye and the length 

 of the eye-stalk, as compared with the length of the body, in the first and fourth larvae, 

 in a lobster 58 mm. long (No. 5, table 32) and in an adult male: 



Table 38. 



Measurements. 



Greatest diameter of eye 



Length of eye-stalk 



Length of body 



Ratio of diameter of eye to total length 

 of body 



First 



Fourth 



No. 5, 



Adult 



larva. 



larva. 



table 32. 



male. 



mm. 



mm. 



mm. 



mm. 



0.74 



0.8 



3 



7 



.92 



1.2 



3.3 



10 



8 



14.5 



58 



264 



.092 



.055 



.052 



.026 



The diameter of the eye, expressed in terms of the total length of the body, is 

 much greater in the first thau in the fourth larva, but is relatively twice as great in the 

 latter stage or in a lobster 2 inches long as in the adult condition. (See p. 163.) 



Labrum and metastoma. — Both the upper lip and paired metastoma have in the 

 first larval stage (fig. 54, plate 28) the general form and appearance which they finally 

 possess. The surfaces of the latter abound in setae. What look like setae occur also 

 on the labrum, but none are present in the adult organ. (See p. 133.) 



First antenna. — The first antenna is a simple appendage up to the time of hatch- 

 ing. In an embryo about four months old (fig. 107, plate 35, and fig. 27, plate 17) 

 it is tipped with short setae and shows no trace of segmentation. When the embryo 

 is five weeks old the first antenna has the appearance shown in figure 77. In the 

 first larva this appendage is no longer simple, as described and figured by Smith (182), 

 but the inner, secondary flagellum (plate 27, fig. 40) is present, though a small rudi- 

 ment, and bears at its apex a single plumose seta. When the stalk is examined from 

 the under side we can detect traces of segmentation into three parts, but on the upper 

 surface the proximal cuticular fold only can be seen. The appendage terminates in a 

 small bunch of setae, one of which is conspicuous for its leugth. It is possible that in 

 some cases the flagellum is not liberated until after the second molt, as described by 

 Professor Smith, but none such were observed. These appendages are immobile in 

 the first larval stage. 



The superficial changes which take place in this appendage during the first five 

 larval periods are illustrated in plate 27, and will not be described in detail. 



In the second larva the segmentation of the stalk into three joints is sharply 

 defined and the flagella show faint constrictions. The clusters of olfactory setae, which 

 increase in length and number with every molt, are developed during the first larval 

 period and appear full-fledged immediately after the second molt. 



The auditory pit becomes prominent after the third stage. In the fourth larva 

 (fig. 43, mi) it is a wide and shallow, -shaped depression, marked with brown pigment 

 cells, bordered with short setae, and containing a few otoliths or granules of sand. In 

 the fifth larva (fig. 44) the closure of the auditory sac has already begun. The j)it is 

 filled with otoliths and the irregular orifice is guarded by short, feathery setae. The 

 constriction of the opening continues until in the adult state it becomes a small pore, 

 into which it is barely possible to insert the point of a pin. 



The first antenna of the European lobster, as represented by Sars (175, tab. I, 

 fig. 4), agrees essentially with that of Homarus americanus, but the secondary flagellum 



