3°8 DISCOVERY REPORTS 



wards dorsally, these muscles extend at wide angles to them. Contraction of the muscles will result 

 in outward rotation of the coxae so that their gnathobases separate. The posterior dorsal muscles 

 originate near the anterior dorsals and are inserted on the posterior margins of the coxae. On con- 

 traction they will rotate the coxae inwards, bringing the gnathobases together. It is the transverse 

 musculature which is so complex. Elements of it originate on various parts of the anterior hypostomal 

 apodeme, or intermandibular tendons fused to it and on the antenno-labral apodeme. These are 

 inserted on various parts of the coxae. The majority will rotate the coxae inwards, giving a powerful 

 biting action by the gnathobases. 



The coxal gnathobase is, as has frequently been pointed out, complex in structure. For the purpose 

 of description it may be considered in four parts. Curving into the oral atrium distally, immediately 

 above the posterior labral shelf, are three parallel incisor edges (Figs. 4, 5, Fig. 8, c.iri). Although, as 

 Claus (1891) pointed out, the structural pattern is more or less specifically constant, there is in 

 Conchoecia antipoda, as in other species, some individual variation. Broadly speaking, the distal ridge, 

 which extends the full width of the gnathobase, is, at least posteriorly, clearly divided into teeth. The 

 middle and proximal ridges are much less regularly toothed, but always bear a long tusk-like posterior 

 tooth. Dorsally to these ridges, the surface is shallowly concave. At the base of this depression there 

 is a ridged pad, the masticatory pad (Fig. 5, Fig. 8, m). Skogsberg (1920) describes this as being covered 

 with fine papillae placed close together. Muller (1894) more correctly refers to it as bearing isolated 

 conical spines. These short sharp spines are so closely packed that the surface formed by their tips 

 does in fact give the impression of being papillose. It is only by focusing through their depth in a 

 whole mount, or seeing them in section that a true picture of the structure is obtained. The third 

 region is the posterior margin of the gnathobase. Here there are four stout spines arranged in a dorso- 

 ventral row (Fig. 8, m.c). These curve into the oral atrium and radiate slightly along the length of the 

 row. Posterior to these teeth there is a dense group of radiating ' needle spines ', each of about the 

 same length as one of the teeth (Fig. 8, tn.f). The fourth structure is a dorsal group of fine spines, 

 almost continuous in distribution with the posterior group, but directed towards the mouth and 

 projecting slightly into it. 



The basis articulates with the coxa so that it extends forward almost at right angles to the latter 

 (Fig. 4 A, Fig. 5). It is clear that its main movement is through a dorso-ventral arc, but rotation of the 

 coxa about its axis will of course move the distal end of the basis laterally inward or outward. On the 

 inner face of the basis, near its distal end, is a fine seta of moderate length, which extends inward across 

 the labrum. At the anterior edge of the gnathobase are two setae, one long and one short, which 

 extend ventrally. On the ventral surface of the gnathobase itself, are two further closely adjacent 

 setae, which also extend ventrally and somewhat posteriorly. Near its distal end the basis bears 

 dorsally a small mamilliform appendix (Fig. 5 ex, Fig. 7, tnn.e), with a long anteriorly directed seta. 

 Muller (1890, 1894) and Skogsberg (1920) have referred to this structure as representing the exopod 

 of the appendage. 



Proximally and ventrally a flange of the basis extends inward to form the gnathobase (Figs. 4, 5, b.in; 

 Fig. 8). This gnathobase lies ventral and parallel to the incisor edges of the coxal gnathobase. It is 

 separated from the latter by the backwardly projecting labral lamina (Fig. 8). The basal gnathobase 

 consists of a single sharp biting edge formed by a row of six shouldered teeth. It is followed posteriorly 

 by first an isolated spine, then a short, stout, spine-like seta. Ventral to the incisor edge is a single 

 serrated tooth (Fig. 8, v.i). On the posterior ventral surface of the gnathobase are a series of short, stout, 

 bristles. The basis and coxa articulate with each other by a fulcrum, which occurs about midway 

 between the distal articular process of the coxa and the upper surface of the joint (Fig. 4 B, b.c). It is 

 clear that rotation of the basis about this fulcrum will produce a backward and forward movement of 



