CALIGOIDEA. 13|7 



alate expansion of the shell of the second or third posterior cephalo- 

 thoracic segments. In the Pandari (see Plate 95), some of the forms 

 assumed by these segments are shown. The first of these three seg- 

 ments is- prolonged backward on either side, and this prolongation, 

 while it is of ordinary character in the P. brevicaudis, has peculiar 

 shapes in the P. satyrus and P. concinnus ; the other two segments 

 are two-lobed behind, and the lobes have different forms. In Lepi- 

 dopus (Plate 95, fig. 5 a), only the last two or three segments are seen, 

 and the lobes are very large. In Dinematura (Plate 95, fig. 4 a) 

 there are also but two ; and while one is small and transverse, the 

 second is very large, and divided nearly longitudinally into two 

 halves. 



The anterior segment, when the semi-articulation exists just anterior 

 to the first pair of natatories (see fig. 1 a! and h of Caligus, Plate 93, 

 and also fig. 2, Plate 94, of Argulus), is not entirely crossed by this 

 articulation ; but towards the middle of either half it bends backward 

 and runs to the posterior margin ; at the same time, a similar pseudo- 

 articulation, or line admitting of flexure (e, fig. 1 h, Plate 93), ex- 

 tends forward towards the anterior antennas, though seldom reaching 

 more than half the way. These lines make a kind of a letter H on 

 the back of the animal, as shown in the figures. The object of these 

 semi-articulations is to enable the animal to draw the margin of the 

 shell down to the surface upon which it may lie, for the purpose of 

 close adhesion. 



A long osseous process lies in the shell of the alar pieces, and en- 

 larges a little at its termination (e, fig. 1 7i, Plate 93), against the 

 anterior branch of the longitudinal semi-articulation ; its object is to 

 afford a firm articulating surface for the purposes of the flexure which 

 here takes place. 



Besides the transverse semi-articulation in Caligus, near the middle 

 of the cephalic segment, there is another a little less distinct near the 

 anterior margin ; it separates a frontal segment which bears laterally 

 the anterior antennas, and is normally the first antennary segment of 

 the body. At the middle there are often two small papillas. Like 

 the longitudinal, it favours the attachment of the animal by this 

 margin. For this movement there are strong muscles, and also 

 towards either side (at c, fig. 1 h, Plate 93), an osseous articulating 

 surface, forming the termination of a process lying longitudinally in 



330 



