LABRUM AND MACULAE 



233 



the doublure is not separated from the lateral parts, or from the 

 dorsal part of the cephalic shield. 



The " labrum " or " hypostome " is attached to the doublure in 

 front (Fig. 137, B, «) ; it is commonly an oval or shield-shaped 

 plate, but is occasionally nearly square. Its surface is sometimes 

 divided into two or three areas by shallow transverse grooves 

 (Fig. 141, A).. Just behind the middle of the hypostome, or 

 when transverse grooves are present either in or near the anterior 

 groove, there are often found a pair of small patches or " maculae " 

 which are more or less oval or elliptical in outline (Fig. 141). 

 The maculae may be (1) surrounded by a raised border, or (2) in 

 the form of pits, or (3) raised like tubercles. In some cases the 



Fig. 141. — A, Hypostome of Sronteus polyactin, Ang. showing maculae, x 4. 

 ma.c\ila, of Bronteus irradians, LmAst. x 12. (After Lindstrom.) 



B, Left 



entire surface of a macula is smooth and glossy ; in others either 

 the whole or a part is covered with granules, and in the latter case 

 the granules may be limited to the internal third (Fig. 141, B) or to 

 the central portion. Sections of a macula show that the granules 

 are really globular lenses similar to those of the compound eyes 

 on the dorsal surface of the head. Some of the maculae wliich 

 are without lenses show no structure, but in otliers there is a 

 spongy or irregularly polyhedric structure with prisms, resembling 

 the marginal zone of the prismatic eyes of some genera. There 

 ■seems no doubt that the maculae with lenses are visual organs, 

 and those without are degenerate eyes. They occur in some 

 genera which, according to Lindstrom, are without eyes on the 

 dorsal surface. Maculae do not appear to l^e present in other 

 Crustacea, but they have been compared with a median organ, 

 found just in front of the hypostome in Branchipus} Maculae 



' Spencer, Geol. Mag. 1903, p. 489. 



